I'm not sure if there is a specific discount, but Brentacre (01792 650933) is run by CGTI members and are very good, and usually very competitive.
VolksWorld insurance run by HIC (Herts Insurance Consultants) offer I believe a 5% disc to members. I paid about 350 TPFT with protected no claims, the good deal with them is that they allow upto 1500 of body mods plus 10BHP power increase as standard part of cover. And if you take comprehensive cover you get free track day cover included.
i've tried all the ones that do discount etc and they're still too expensive! i just got a good deal with Privledge.
I have just gone with Brentacre. Most of the companies I phoned wouldn't touch me because of my modifications. Direct Line say you need to be 25 to have mods! How does that work? So Brentacre it is, 810 fully comp, with protected NCB.
