They do it to bikers all the time mate - If the pipe doesn't have a Kitemark or CE stamp, they say it is illegal. They actually have the power to unbolt it and confiscate it at the roadside if they really wanted to.

Think you'll be on to a loser with this one myself. Best just do the MoT and put it down to another highway robbery by the government.

One that always cheers them up after they do you for something like that is to say "For a minute I thought you were pulling me up to tell me you caught the guys that burgled my house last year, but I can see you obviously have higher priorities...."