The update is that the service centre's insurance have told us to claim on our own insurance. So to answer one of the previous questions, as far as customers are concerned they may as well have had no insurance at all.
So it could be bye bye no claims and excess, but at least we are slightly more in control of the process.
Regarding securing the engines, you could have 4 locking bolts but they will either chop the transom, as said, or they'll probably leave the engine bracket on the boat!
One of the other boats had a cheap locking bolt on one side and for some reason they decided not to bother trying to shift it, so they undid the two bolts on the other side and managed to take the engine and one half of the bracket, leaving just the other side of the bracket still bolted to the boat!