The Mail on Sunday on 4 March this year had a very interesting article about holiday insurance for the over 65s and those with pre-existing medical conditions (not sure if you would be able to access it on line or not. The companies they quoted for the latter were:
www.allcleartravel.co.uk, www.free-sprit.com, www.marrs.co.uk and www.medi-cover.co.uk and Holiday Services (01773 769406.
Haven't checked through the previous answer's contacts so they may be repeated, but too much info is better than not enough!!
The very best of luck with your search.