This specialist broker might be able to help. (They've helped several AB members in the past):
http://www.able2travel.com/over65.html
(Although their own policy only goes up to age 79, they're probably able to source others for older customers).
I used the all clear website and they recommended OK TO TRAVEL and I found them quite reasonable. They are a new company that were started especially for people with pre-existing conditions. I found them a lot cheaper than the one I used last time.