Just for information I have just changed to Lancaster Insurance which is linked to the MG Owners Club. The premium is £138 for fully comp / 3,000 miles and includes £18 for the agreed valuation by the MG Owners Club. The agreed valuation for my 1970 Costello MGB V8 roadster is now £30,000. Two other Costello owners have got similar valuations via Lancaster Insurance. This agreed valuation is quite a jump from my previous insurer - that was £15,000 (done in 2016).
I do not know if Lancaster are a good or bad insurance company when accidents happen however they are promoted by and have links to the MG Owners Club of which I am a member. It shows that one has to keep an eye on agreed valuations and make sure they are up to date.
Thanks Harry. Good advice. For info I use a different insurer; Carol Nash (I have old motorbikes too) and, their agreed valuation on my GT Costello, is around the same. Done without any further discussion or information needed.