Dealers are now obliged to register online all vehicles being run on the roads under their trade policies after a few days possession, and to remove them as soon as sold. Anything registered online will appear on the DVLA databank as insured.

There are two matters here: one is whether you personally were insured by the garage to drive cars on its trade policy; the other appears to be a refusal to cover your car. This cannot be covered by the garage trade policy because it isn't their car. It's your car.