Build Day 155.....Bit of a Disaster

Tried to get the engine in yesterday. All seemed to go ok, but once the gearbox was bolted on, the engine wouldn't turn over. Took it all out to find that, for some reason, the gearbox input shaft had not meshed with the clutch and instead had pushed in and bent the clutch friction plate - which is now scrap.
What's worse is that I now think that the alloy backplate, which is attached to the engine, and is what the bell housing bolts to, has been bent out of shape too (by bolting it all up when the input shaft hadn't located properly).
I'm going to bolt the box back to the engine on the bench without a clutch in to see if it's all still true. If not, I'm going to either have to find an new backplate, which may be impossible, or get a new engine.
Not a good day.