Same happened to me years ago,about 10 firearms, all deacs with certs!!
Story is I was selling some German stuff to a so called collector. He wasn't really interested in what I WAS selling, more of what I WASN'T! Kept enquiring about the weapons, offering stupid low prices for them. Finally had enough of him and told him to leave. His final words were, "it's illegal to own them and call me if you need to sell!!"
Next day go into work and redirected to two CID officers enquiring about illegal weapons and wanting to see them.
Got a nice ride in police car home, freely showed them weapons, certs and even stripped them to show ground off bolts and welded bores!!
NOPE, still illegal according to them!!! WHAT?????
So World Wide Arms selling illegal weapons now???
Politely asked where they got this stupid info from and was told by a concerned member of the public!!! Guess who???
They even called the Rapid Response guys who came tooled up.
Told them the same, showed them the same and still no clue!!!
Had them taken away for further investigation.
Took about two months of phone calls and visits to the station until told, I can go collect them!!
Why can't they return, too busy was the answer.
So I sympathise with you mate. Mine was in the late 90s, nice to know they still don't have a clue!!!
We do public displays now and always make the point of notifying the local plod.