Austin Champ Trailer Insurance

Trailer Insurance
A trailer towed by an insured vehicle is covered for Third Party claims against the policy holder, say, if the trailer breaks away and causes injury and\or damage to other people and\or their property.
But if a trailer is damaged or stolen it is not covered, even under comprehensive insurance, unless the insurance company has specifically agreed to insure it.
Values of Champ trailers. 10 to 15 years ago a Mk 2 Sankey or Brockhouse trailer in good condition could be bought for £400\500 or even less. In 2013 there are fewer and fewer Mk2 trailers up for sale. The Mk 1 trailer with the impressed crosses on the body is even rarer.
I decided I wanted to insure my Mk1 trailer for damage or theft. My vehicle insurance was due for renewal so I telephoned around the insurance brokers for quotes and many of the quotes were cheaper than the insurance with my existing broker but, as soon as I mentioned insuring the trailer for an Agreed Value* all the brokers said they could not do it.
*Agreed Value - an 'Agreed Value' is the amount previously agreed with an insurance company and will be payed out if the vehicle and\or trailer is damaged beyond economical repair or stolen. This is different to a modern vehicle policy that pays out the depreciating market value only.
I spoke to my existing broker, Footman James, and they were not sure if they could do it but came back to me and said they could. I had to give the chassis number, fill in a condition form and send photographs of the trailer and it is now insured for £1,500.
My Mk1 trailer is in very good structural condition but is due for a complete strip, refurbish as required, shotblast and repaint. If I got a vehicle restorer to do this work I would estimate the cost at, possibly, £3,500. An Agreed Value should approximately reflect the (appreciating, hopefully) replacement cost of the insured vehicle and\or trailer, Footman James accepted my valuation of £1,500 but if I did, say, spend the £3,500 could the Agreed Value be increased to this?, I would possibly doubt it could so if a lot of money is spent on restoration then it is a matter of negotiating an Agreed Value with the insurance company.