Wow that mustve been an interesting project, mind you a ton of paperwork and conservation officers breathing over it.We use Trade Direct insurance services
If you talk to them they will taylor your policy exactly to your own requirements. Can also increase your liability limits for short periods for particular jobs.
We recently did a large contract on a grade 1 listed building including several weeks leadburning. We were able to go from our normal £6 million to £25 million to cover house and contents for a 3 month period.
Your company must be pretty specialised to get that sort of work