Has anyone had trouble here buying an agreed value policy and then a month later getting the paperwork through with an agreed value 20% less?

I pointed out what I thought was an error and they replied saying that they might reconsider if I can supply an engineers report (not the owners club valuation they were happy with before) or seek an opinion from some outfit called www.classiccarvaluations.co.uk that I've never heard of but charge a minimum of £95 for a "desktop valuation" that might or might not be acceptable.

I've suggested that this is a fundamental change and I should be allowed to cancel with a full refund becuase I'm saying so within days of them communicating this change. Am I right or will I just have to grin and bear it this year?

I've always been a supporter of Flux but this feels like sharp practice and I am REALLY pissed off with them to the point that if it isn't sorted I will never deal with them again.

Any recommendations for another insurer for a very modified car on a classic agreed value policy?