In reply to EddInaBox:
I only received the email at close of business hours last night, so it was too late to be able to call anyone. My plan this morning is to find an avocat here in France who specialises in fiscal matters and then possibly go amd have a sit-in at the accountancy firm until they sort it.
I have a contact number for one of the company directors, so I will be calling them this morning as well, but knowing how they work, I don't expect to get very far.