Right then, an update. As suspected, driver side bolt failure. One not present, another sheared but still in place, and another with half not present, the other half sheared flush with the block. So the engine is supported by one bolt on the driver side, and it's sitting on the ARB. No damage to the ARB. The car is sitting with the engine supported on a jack to relieve stress in their workshop. The mechanic said it was lucky I know my car so well and brought it in, as if the other bolt had failed something catastrophic could have happened
I have asked them to take some photos if they remember, though TBH I'm more concerned about the car being fixed.
Because of the stress on the gearbox and other engine mount bolts he's going to replace them all, and probably put new OEM mounts in too whilst they're in there. Happy with that as I want to feel the car stock when sorted before buggering about with anything else.
I've no idea how much it will cost yet, mainly because they don't know if they can get square on to the bolt sheared in the block yet. If they can't they will have to drop the engine and transmission out to sort it. Just saying that makes me wince. I've not even thought about the 'I told you so and you told me everything was fine' conversation yet, as it's possible the bolts looked fine the last time they looked. Though he did say it was exactly what I expected.
On the plus side, it's been caught in time and can be fixed.
Don't forget that this car has not been abused either - yes it's been driven hard a lot of it's life as a pleasure car, which is probably a contributory factor, but it's never been tracked or crashed and is always warmed properly... Yet this has happened. Definitely something for everyone to check.
I'll let you all know how things go over the next few days.