Try Besso in London. Frank Bannister from the aviation department has been in the industry for years and knows many of the underwriters well. He knows which underwriters to approach based on aircraft type, pilot experience and history etc. Although he may not necessarily produce the cheapest quote, if you ever need to make a claim, he's the guy I'd want in my corner. He works for the policyholder, not the underwriter!
In insurance, perhaps more than anywhere else, you really do get what you pay for.
JJ