Thanks for the replies. It is (or was) a petrol engine. The problem with asking for any reports, and the other similar suggestions, is that they are simply ignoring all written communication. It seems pretty clear cut to me though - not only was it running perfectly when I took it in, but they actually took it out for a test drive themselves and said there was nothing wrong. The head has already been taken off, damage definitely caused by a foreign object according to the engineer (I haven't seen this myself, but he has pictures). Seems like my best option is to try one more letter, recorded delivery, and assuming they ignore that too then to start a county court claim.

I'll try and make this as brief as possible... Drove my car 90 miles to the office (not a daily commute!), everything absolutely fine and running perfectly, except a minor judder on the front brakes at high speed, which had been going on for some time. As there's a National Tyres branch just down the road, I thought I'd leave it with them to get the brakes checked. Not long after, they phoned to say they'd test driven the car and the car was fine - in particular, nothing wrong with the brakes. I said while they had it, could they give it a minor service - i.e. change oil, plugs, filters, etc. The next thing I know, some hours later, I get a call saying there is a problem - no compression on cylinder 1. I arrive at the garage to find the mechanic with the rocker cover off. He shooes me away into the waiting room "so he can take a proper look". The end result was driving a perfectly running car in, and taking it back home on the back of an AA truck. I then got an engine specialist to recover the car from my house to investigate, which he did first with a borescope, and then removing the head. There is serious damage to the surface of the piston, and the valve is blown out. He's convinced they caused this, almost certainly by dropping a foreign object into the piston. The branch manager at National Tyres refuses to discuss this. He says he "1000% guarantees" they didn't damage the car, and he's "too close to it" to talk about it any further. However, repeated messages to anywhere higher up the chain have been completely ignored. The car is basically a write-off - most cost effective repair is a complete engine replacement, which is more than the car is worth. So I am down for a) cost of replacing car, b) cost of recovery and investigation, c) cost of hire car while replacing car. What's my next move? Given that they refuse to discuss or respond, do I need to go straight to a small claim?