What I would do is run a quote through on an internet quote engine, pretending that you have maximum no claims bonus. (someone like Norwich Union Direct) This will give you an idea on what premium you should be paying.
Then telephone that particulary insurer, explaining that you want a quote and ask if they offer the equivilant no claims discount for company car experience. Then see what happens and make comment to the quote you have if it differs. Online insurance is really easy if everything is straight forward and sometimes actually speaking to someone can have really good results. I see you went to a high street broker - was this a mainstream or a small independent? You will be surprised what deals a small local broker can have access to sometimes!
To be honest a good broker should be getting you 60% discount if you have evidence of 4-5 years claim free under your companies previous fleet insurance.
One final suggestion would be to try an owners club type insurance scheme if one is in existence.