Adrian,
This is from my personal experience only but have a look for a company called Noel Dazeley. They're an insurance broker, and are quite happy dealing with even heavily modified cars, should you so wish. They sorted my policy out for Holly my LWB (which is just slightly modified) and for fully comp came in at not much over £300. Admittedly, it may not be for everyone, but I've got a proper 4x4 policy that covers me for laning and P&P days, has a pretty sensible mileage figure for the year, and when I started mentioning lifts, they asked me if it was body, suspension, or both - good sign I think, given some other companie's ineptitude.
Only slight downer is that I don't have other vehicle 3rd party cover, but that's something I can live with. Like I said, not everyone's taste.
Jim