They've since said I can get my garage to fax them the lower quote and although it's unlikely there's a possibility they may accept it. In this case my garage is now quoting new parts so the only difference between the £1,500 estimate just received from my garage and the £2,900 one received from their approved supplier is the new roof supplied in the latter.

Unless they refuse to accept it I'd rather retain my non write-off status.

Do you think their refusal would be acceptable [i.e. in line with custom and practice in the insurance trade] ? Thanks for your advice thus far.