The requirements for hazard warning lights are in the Road Vehicle Lighting Regs, rather than the C&U regs.
Sorry, obviously you're right that everything related to lighting is in the Road Vehicle Lighting Regs. rather than C & U Regs., although of course the latter covers a lot of legal requirements.
I certainly think it's reasonabl to have a requirement that safety-related things which are fitted have to work though!
But the point here is that there is no such legal requirement. For example, if the regular turn signals or brake lights on your 1976 car aren't working, that makes it legally unroadworthy because they're a requirement of the Lighting Regs. for a car of that age. But if the hazard warning lights on that same car are fitted but not working, it's still perfectly legal to drive it on a public road because they're not legally required, even though it would be an MoT failure.
And I'm not sure it would actually be reasonable to require a device which is fitted to be working even if there's no obligation to have that device in the first place, which is why the MoT really goes too far in some instances. It's also inconsistent since while it requires hazard flashers to work if fitted regardless of the age, it doesn't, for example, require all rear fog lights fitted to be working, only the single one required for cars first used around 1980/1981 onward (at least unless there has been any change to that in the last few years).
I've learned something there! I didn't know that on cars fitted with two rear fog lamps, only the offside one needs to be working! However, it IS illegal to have a car fitted with hazards that aren't working. The MOT can't (at least, I'm pretty certain it can't!) demand anything over and above the legal requirements. The Lighting Regs state:
"Maintenance of lamps, reflectors, rear markings and devices
23.
(1) No person shall use, or cause or permit to be used, on a road a vehicle unless every lamp,
reflector, rear marking and device to which this paragraph applies is in good working order
and, in the case of a lamp, clean.
(2) Save as provided in Paragraph (3), Paragraph (1) applies to–
(a) every–
(i) front position lamp,
(ii) rear position lamp,
(iii) headlamp,
(iv) rear registration plate lamp,
(v) side marker lamp,
(vi) end-outline marker lamp,
(vii) rear fog lamp,
(viii) retro reflector, and
(ix) rear marking of a type specified in Part I of Schedule 19, with which the vehicle is required by these Regulations to be fitted; and
(b) every–
(i) stop lamp,
(ii) direction indicator,
(iii) running lamp,
(iv) dim-dip device,
(v) headlamp levelling device, and
(vi) hazard warning signal device,
with which it is fitted.
(3) Paragraph (2) does not apply to–
(a) a rear fog lamp on a vehicle which is part of a combination of vehicles any part of which is not required by these Regulations to be fitted with a rear fog lamp;
(b) a rear fog lamp on a motor vehicle drawing a trailer;
(c) a defective lamp, reflector, dim-dip device or headlamp levelling device on a vehicle in use on a road between sunrise and sunset, if any such lamp, reflector or device became defective during the journey which is in progress or if arrangements have been made to remedy the defect wi th all reasonable expedition; or
(d) a lamp, reflector, dim-dip device, headlamp levelling device or rear marking on a
combat vehicle in use on a road between sunrise and sunset."
(I included that last paragraph as that's the bit that I thought I'd seen where it cuts you a bit of slack if the light concerned packed up during that trip..."officer..." (!)
There are other parallels in the MOT. Some of the very first cars to be fitted with cats need to meet cat emissions requirements, even though other cars of the same age don't. My own car is old enough to need dim-dip headlights, so it needs to have its dim-dip working even though its no longer a requirement for new cars OR cars that were more than about 5 years older than it. I've always felt that was really unfair!