Just to check - I am right that letting the IMCR lapse only means I can't use the IMCR privileges until I've taken the test? (Assuming the lapse is only a week or two.) No retraining required?
Yes, that's correct.
Instead, I did a type conversion onto the club's new Firefly
That's ok, so long as it was at least an hour, and the Instructor countersigned the entry in your logbook.
MJ