Having waited in a layby on the A46 for three hours I was eventually recovered by the AA and had the car taken to a Mercedes specialist in Bristol. Upon closer inspection it needed a lot more work than replacement tires - the rear brake caliper was seized on, brake lines were seriously corroded, the rear left suspension spring had snapped, the damper was shot, the rear sub frame bushes were badly deteriorated, there was extensive rusting on the exhaust which I had patched many times. Oh, and the head gasket has blown. It could also have done with a respray.
All in all, it was going to take an awful lot of money to get the car running again so I reluctantly decided to cut my losses and scrap the car. Quite how it "passed" the MOT is beyond me.