try Free Spirit on 0845 230 5000 / www.free-spirit.com
or
AllClear [ for cancer patients it says] on 0870 777 9339 / www.allcleartravel.co.uk
You will have to talk to them personally at some stage.The popular insurers that advertise on tv etc dont do specialist insurance and firms like Diabetes UK , Marrs Insurance Brokers and MediCover are all incredibly high, but then so is their risk.
The first two should be around the �200 mark, good luck, I had to do this in April for a lady with Downs Syndrome and a heart condition it took me nearly 6 days and hours on the phone only to be told after a 30min conversation that the answer was either no or the premium was unafforable, it takes a lot of patience so good luck.