Most "agreed value" policies I've ever had, require an independent valuation from a recognised owners club, along with photographs (non digital) to be supplied upon inception of the policy.
Further to this, the "agreed value" normally stipulates that the cost of any "restoration" is not necessarily factored into the valuation process.
So to sum up, the MGOC will be able to supply the valuation, but it may not necessarily match your expectation of perceived value, or indeed reflect the monies that have been spent.
Also be careful in quoting the sale price of "similar" vehicles, as the insuring underwriters are not likely to equate a low mileage diesel ZT to that of a high mileage petrol ZT based upon the only common factor of the cars being the same colour when ordered from the factory.
I've always found this to be a problem in the past, where restoration costs far outstrip the vehicle's worth, and that is even where labour costs are not a consideration.
Hopefully you will find a solution Bob, have you contacted your current insurer to see what their take is on the situation, often a phone call or email will solicit the information you need
Brian