I'm with Gott & Wynne and pay £470 for the ARP4, I was told that that if you have a accident on track the excess is 10% of the value of the car.
Interestingly I was going to borrow a Plus 6 from Krazy Horse and had to get my own insurance which is fair enough, however I couldn't get insurance as I'm under 50!
I said that a bit strange as I have a tuned BMW 140 with the same engine and insurance is only £600. I'd like to try one but might have to wait a while haha.