Are you actually a trader?
A friend of mine used to have a trader policy because he had several cars and it was the cheapest way to cover himself for them.
This was fine for a few years until one day they decided that they wanted proof of him being a trader, such as receipts and stuff. Of course he couldn't provide it so he had to switch to a private policy. However, because his NCB had been built up on a trader policy he couldn't get any NCD when he switched over.
He, er, wasn't pleased by this.