The current version of the RVLR was published in 1989 , then amended in 1994 , 1996 , 2001 , 2005 , again in 2005  and most recently in 2009 . So it can be a bit of a puzzle to work out exactly what applies here and now!
It has to be said that the fine details of RVLR are seldom enforced, and provided you show some kind of white light in front and red behind, you are unlikely to be challenged. If you are involved in a night-time accident however, any slight illegality with respect to your lights or reflectors may be regarded as contributory negligence.
The main points of RVLR are as follows:
- Lights (and reflectors) are required on a pedal cycle only between sunset and sunrise.
- Lights (and reflectors) are not required when the cycle is stationary or being pushed along the roadside.
- When they are required, the lights and reflectors listed below must be clean and working properly.
The following items are the minimum required, on a bicycle or tricycle, in order to ride it legally at night:
One is required, showing a white light, positioned centrally or offside, up to 1500mm from the ground, aligned towards and visible from the front. If capable of emitting a steady light, it must be marked as conforming to BS6102/3 or an equivalent EC standard.
If capable of emitting only a flashing light, it must emit at least 4 candela.
One is required, to show a red light, positioned centrally or offside, between 350mm and 1500mm from the ground, at or near the rear, aligned towards and visible from behind. If capable of emitting a steady light it must be marked as conforming to BS3648, or BS6102/3, or an equivalent EC standard.
If capable of emitting only a flashing light, it must emit at least 4 candela.
One is required, coloured red, marked BS6102/2 (or equivalent), positioned centrally or offside, between 250mm and 900mm from the ground, at or near the rear, aligned towards and visible from behind.
Four are required, coloured amber and marked BS6102/2 (or equivalent), positioned so that one is plainly visible to the front and another to the rear of each pedal.
Exceptions and explanations
Age brings privileges. To name but two: cycles manufactured before October 1990 can have any kind of white front lamp that is visible from a reasonable distance, and pre-October 1985 cycles don’t need pedal reflectors.
Cycle trailers need a rear lamp as above and a triangular rear reflector with an ECE mark III or IIIA. (Trailer manufacturers appear blissfully unaware of this however, and generally fit cycle-type reflectors whilst making no provision for a rear lamp.)
In the absence of any more specific requirement, a cycle-type rear reflector can be presumed okay on a sidecar, which also needs its own front and rear lamps.
Seriously reduced visibility
Unlike other vehicles, pedal cycles are not required to light up when conditions of seriously reduced visibility occur in daytime. That's because this clause applies only to lights that are required to be fitted, and lights are not required to be fitted to a pedal cycle between sunrise and sunset. So even if the cycle has lights fitted and is ridden into a bank of fog for example, they do not have to be switched on. It would nevertheless be wise to do so!
'Lighting-up time' and the 'Hours of Darkness'
These commonplace phrases have meant many different things in the past and continue to cause confusion. Remember: for a cyclist the only thing that matters is the sun, the moment it dips below the horizon, our lights must go on, even though there may be plenty of light to see by for another half hour or so on a clear evening.
Sunset to sunrise: that's lighting-up time, when drivers must also switch on their sidelights at least. They can leave off the headlights for another 30 minutes, until - you guessed it - the 'Hours of Darkness', which nowadays endure from half an hour after sunset until half an hour before sunrise.
Prior to 1989, cyclists could also wait until then before switching on their lights, and if you go back further, the 'hours of darkness' were 60 minutes shorter, starting a whole hour after sunset and ending an hour before sunrise. Go back further still, to before the roads were nationalised, and some counties didn't require lights at all during full moon! It wasn't any lighter back then, but when things moved more slowly I guess we didn't need to see them coming so far off.
Beware that some publishers of 'lighting-up times' are a bit careless and give the 'hours of darkness' instead. Don't trust any such data unless it clearly states the times of sunset and sunrise.
The Euro-friendly clause
Thanks to a European Directive of a few years ago, wherever a British Standard (BS) is referred to, equivalent standards from other EC countries must now also be recognised - but only if they provide an equivalent level of safety. It’s not exactly clear which do. However Germany has arguably the strictest cycle lighting laws in Europe so we consider it safe to use equipment that is marked accordingly, with a “K~number”.
It should also be noted that wherever a British Standard is referred to, that reference applies to a specific edition. In the case of BS6102/3, that is the 1986 edition, as amended on 15th April 1995 and again on 1st September 2003. These amendments (in conjunction with the 1994 amendment of RVLR) removed the filament bulb design restrictions, so that lamps may now get their light from LEDs, HIDs – or whatever comes next!
Dynamo powered lights are legal even though they go out when you stop. That’s allowed so long as you stop on the left. Usually it’s much safer to stay where you are (e.g. in a stationary queue with left-turning traffic filtering up your inside), since most cars do stop for red traffic lights and those that don’t are unlikely to pay more heed to a bike lamp!
Nevertheless, dynamos and lamps are now available with reliable back-up (standlight) features that either keep them on or light up a diode instead of the bulb.
Additional lamps and reflectors
Some cyclists like to fit extra lamps and reflectors, in addition to the approved ones specified above. This is perfectly legal provided they are the correct colour and in an appropriate position.
These optional lamps and reflectors do not have to comply with any standards, but it’s illegal to use some designs of lamp or reflector that have specific other uses. You must not, for instance, show a red light at the front, or a white light to the rear, or fit triangular-shaped rear reflectors on anything other than trailer.
The suggestion that any lamp on a bicycle might cause undue dazzle or discomfort to other users of the road would have been laughable a few years ago, but not any more. It is not only illegal but doesn't actually improve your safety, should the person driving a heavy motor vehicle towards you be temporarily blinded by your headlamp! So please don't let it do that.
The Pedal Cycles (Safety) Regulations  (PCSR) ensure that every new bicycle is sold with several extra reflectors, not required by RVLR, but (strangely) does nothing at all to facilitate the fitment of front and rear lamps.
These additional reflectors are found on the sides of the wheels, clear white or coloured yellow, and there's also a "white" reflector on the front of the bike. You are at liberty to remove the front and side reflectors, but beware that there are some situations (albeit most unusual) where a front reflector might improve your safety, and be sure to fit the necessary front and rear lamps.
Thanks to the 2005 RVLR amendment  on 23 October of that year it finally became legal to have a flashing light on a pedal cycle, provided it flashes between 60 and 240 times per minute (1 – 4Hz).
Even better: it became possible for a flashing light to be approved, meaning no other light would be needed in that position. And since BS6102/3 does not cater for flashing, approval is granted simply on the basis of brightness (as specified above).
Because DfT very much prefer things to be evaluated against a proper technical standard wherever possible: any flashing lamp that is also capable of emitting a steady light is approved only if it conforms with BS6102/3 when switched to steady mode. Since most flashing lights do also have a steady mode, they're legal but not approved, so you'll probably need another lamp that is.
Unfortunately Britain is not the force it once was in the world cycle market, with the result that very few manufacturers can nowadays be bothered to test and mark their products to our standards. So it can be really hard nowadays, to find any approved lamp for sale these days, flashing or not!
Fortunately our Police seem hardly more concerned by legal niceties than lamp manufacturers. Since it became theoretically legal to ride a bike with just flashing lights on it, they're nowadays no more likely to quibble its legal status than one equipped with steady lights – unless they're rather dim (the lights that is) or involved in an accident. Then it's possible that someone might look a bit closer.