Hi Roy,
As far as I know the main ones for agreed value for classics bikes and cars are: Carole Nash, Footman James and Peter James Insurance. Peter James was originally the other half of Footman James but split away on his own. Footman James was taken over by Towergate Insurance at some time but still trade as Footman James.
I'm with Peter James Insurance as I remember from years ago that Footman James had a good repurtation but when these companies with a good reputation get taken over things often change for the worse.
I have comprehensive agreed value £6,500, no mileage restriction, both myself and my wife to ride, legal and breakdown cover, excess only £50.00. It's underwritten by KGB. Cost last June was £92.53. If your valuation is under £10,000 you can do it yourself after filling in a condition form and providing photos. They then get it approved by their valuer and if the valuer agrees with what you say they will insure you for that anount. If over £10,000 you have to get a club official to value it for you. They also give a discount for BSAOC membership. I found phoning and speaking to someone was the better way.
http://www.peterjamesinsurance.co.uk/