Oh fart. I have suddenly realised why my Logic programme has gone bananas today. I have been recording a vocal with a different microphone to normal and it's great to sound different, but the program was REALLY playing up, basically doing what it wanted, and in the end I saved everything and stopped.
I've just realised what the problem was- the computer from work for online teaching came with a duff keyboard that wouldn't do things like full stops, and a duff mouse that you had to bash to get it to change anything, so I swopped them with the ones on the recording computer.
THAT's why it wasn't working. Duh.