I'll give a 2nd nod to Peter James. They specialise in older and modified bikes and offer agreed value cover. My three bikes are insured with them - the agreed value on Miss November, my XS650, which has been modified required me to send some recent pictures and an account of the work that has been done. Cost of cover went up this year but I think it's still reasonable at £202 for three bikes, fully comp, unlimited mileage, includes holiday cover throughout Europe, and with agreed value on one of the bikes.
Plus, I have found them down to earth and easy to deal with.