Butt out with your left buttock Mac!
V6 Man wrote:The fact you have the old cert making it ok is a common fallacy.
Not the case. Have a look at the The Motor Vehicles (Tests) Regulations 1981(and for those of a certain age, 1976 (and 1968 or earlier if you can find a copy somewhere)) When a test is "failed" a refusal to issue a certificate takes place. There is no cancelling of unexpired ones.
If caught in the period between failing and the original expiry date, there is no way you could be done under S47 of the Road Traffic Act 1988 (or whatever section of the RTA 1972 in ye olden days)
Section 47 wrote:...A person who uses on a road at any time, or causes or permits to be so used, a motor vehicle to which this section applies, and as respects which no test certificate has been issued within the appropriate period before that time, is guilty of an offence...
It's worded that way for a reason.
That's not to say you couldn't be done under S40A "Using vehicle in dangerous condition" or S41A "Breach of requirement as to brakes, steering-gear or tyres." or S42 "Breach of other construction and use requirements." Even if the plod did throw one of those at you, it still wouldn't be an open and shut case at court.
V6 Man wrote:What's little known is that they prosecute under 'Constructions and use' legislation and not MOT regs.
See above as it's another bit of waffle.

They prosecute under the relevant section of the "Road Traffic Act 1988" not C&U. Perhaps you mean it's a construction and use offence
under the RTA?