Try Keith Michaels as well. They are specialist insurers. I'm sure they take into account your no claims and prior experience on performance cars more than the numpty insurers do.

http://www.keithmichaels.co.uk/

Maybe ask if one of their sales staff specialises in Ford in particular, because Gary Moulson that works there (if you check out the 'Contact Us' section of the site) specialises in Subarus and will get you a better quote than the rest, taking into account mods, etc. I'm sure another sales guy specialises in high powered Nissans, etc, so it may be the case that one of them could give you a better quote for a Cosworth.