It appears that we’ve figured it out – apparently the ticket needed a “Start Sale” and “End Sale” date, and without it it was not showing at all. The help text under that section in the ticket set up says:
When will ticket sales occur? If you don’t set a start/end date for sales, tickets will be available from now until the event ends.
So we’d left it empty, but something must have changed requiring that to now be filled out.
We have managed to get the tickets showing again, but perhaps worth updating the help text if it is now required?