The story has concluded.
All the gear has now been picked up and they've had the drill fixed. They stopped 50 quid out of my deposit - 40 for parts and 10 for labour. I thought it was a bit steep, but since they already had my money, I wasn't in a good bargaining position. If I'd have had more time I'd have argued the toss endlessly but as it was I just grumbled and paid.
Presumably, they had already given up any hope of repeat business...
Cheers,
Colin.