You have had the best advice you are going to get here, (topendtorque's is excellent), now you need a good broker, the City has several, some of whom will specialise in helicopters. Brokers don't cost extra, as some think, they are paid by the underwriter and work for you, they not only bring in the business to the U/W but do a lot of preliminary stuff that aids the U/W.
There is a bit here:
Choosing a general aviation insurance broker - Corporate Jet Investor
Hayward Aviation Ltd, LL Johns and Willis Aerospace would all be worth a try.
The questionnaire here will give you an idea:
http://www.cspinsurance.com/Helicopter_Quote.pdf