You'd be better to negotiate a deal with them to repair both.

undersatnd that mallory but what do you think i should do with vauxhall

For safety's sake I would get all the necessary work done, but don't forget that an MOT is technically only valid on the day of issue. It could pass with flying colours one day, yet tomorrow it could have another test and fail. It's not a guarantee that the car will be legally roadworthy for the full year.

Can anyone give me some advice;
bought a mercedes second hand from our local vauxhall network q dealership january last year.
car looked fine under my inspection. although after first wash noticed heavy front bonnet chipping which wasn't eveident at the time but thats another story.

on completion of purchase full MOT and a minor service was undertaken by them and passed...
12 months later lo and behold failed MOT
reason wrong tyres on same rear axle
wrong width and height and speed

so went back to vauxhall and explained.. after some deliberation they came back to me and said they would replace the offending tyre..great but really need to replace both now as the one correct tyre is in need of replacing.

Dilema: should I push the envelope and insist they replace both tyres. on the basis that i have effectively been driving an illegal car for the past 12 months unbeknown to me, and as their MOT passing station effectively passed a car that is uninsureable in the event of a crash.

Now I am knowingly driving this car in this position...

should I push the envelope????
not much good at this kind of thing ...but sick and tired of coming off second best..cheers