Why not PM Tim above, they worked some serous magic to insure a custom build transplanted V8. A lot of hoops had to be jumped through, paperwork, referrals etc. Not only that, but they wouldn't insure me unless they were confident the work was done to a specific standard.
They have never failed to insure me, with specifics such as merging a modified and un-modified policy into a multi car policy. It might take a few days, but leave them to work something out for you.
It's was prestige that quoted me. They said they'd only returned one underwriter that would touch me but not cover the mods on like for like.