I have complex medical conditions and used to be with Staysure. I changed from them, as every time I phoned to note a change of medication , (and it happened often), they charged me an admin fee.
The agent would raed from a script and would not deviate from it. It drove me mad.
On the recommendation of a friend, I went with All Clear and they were almost half the price. I had to cancel a trip, due to an unexpected stay in coronary care and got my money back. I found them to be very helpful.
However, when I phoned to add my new heart condition to my annual policy, they said they were no longer able to insure me, ( medical condition overload!) and cancelled my policy there and then.
I didn't get a refund of the remaining premiums, as I had already made one claim.
I went on a comparison website and Saga are the only company who will now insure me for my forthcoming trip to New York, I almost needed a mortgage to pay for it. It cost more than the actual trip, but is on my "bucket" list and I know my days of going abroad are numbered. 
You must be really honest about your conditions or visits to the doctor's, no matter how trivial, or it may invalidate your policy.
I always worry that I may have forgotten something.