Thanks to all who made suggestions and recommendations. I've just logged in after a very long gap. I thought I'd add an update. I decided to spend no more on the car and sold it to a mechanic who planned to fit a used engine. I decided not to take the matter further (I'd considered taking the garage to court) when the owner of the garage wrote to me denying that they'd ever said ever that there wasn't a problem with low compression. The fact is that they did say that. They said that they'd tested the compression and it was fine. I took the car to a different garage and was told that one cylinder had zero compression, which is just was the RAC guy said at the roadside.