Sounds like Broker bull to me. I used these 10 month policies to rebuild my NCB after a theft claim.

I have moved to a 12 month with another insurer and they said NCB from a 10 month is no problem.
When you get your proof of no claims from these 10 month policies it doesnt say whether it was earned on a 10 month or 12 month.
It does show the dates for your proposed renewal so all you do if you want to hide that you were quoted on a 10 month is ring up and get a requote for 12 months, once that has been processed ask for a new proof of no claims as you misplaced yours. Hey presto a NCB proof certificate that shows renewal dates covering a 12 month period. Seriously dont worry these are a good way of building up NCB pretty quickly. It worked for me.