Sorry folks, been away this weekend...update is...
Car is back
And feels good as new (but maybe that's in comparison to driving a Passat 2.0 diesel!).
Anyway, they dropped a courtesy car (the Passat) at my house on the Monday, and during the week fitted new mounts (both sides, at my request), new cambelt, offside cambelt cover (apparantly), did whatever they do (compression tests etc) to confirm all working OK, and delivered back to me on Thursday.
No, I never did get a "better" answer than "the bolts sheared off", as they said they were too damaged to determine the cause (they said they could have been faulty, or not secured properly). I wasn't there when they looked at it (I had to drop the car off on the Bank Hol weekend, and then I was 20 miles away at work on the first working day afterwards (Tues)), so couldn't examine them myself, and they dropped the car back for me as well.
The service manager said he will send me a photocopy of the internal recharge invoice (not sure to whom?) that details what was changed etc, and presumably (?) will say why they had to incur said cost. I will chase if I don't get this.
So overall, of course I'm not happy that it happened (obviously). But I do think they dealt with it well (so they should, some might say), and can't fault them for doing the work with no quibble, keeping me informed, getting me a courtesy car etc. And they also did a small amount (£30 worth) of additional work that was totally unrelated and didn't charge me.