This October I told my Travel Insurance company that I was due to go into Hospital a week after my long weekend in Boston for a Cardioversion.
I had a annual cover with Staysure. Well that threw them and had to refer to underwriter. I was then refused the cover, for everything.
I was told to ring BIBA, as they will help as they have info on companies that will insure you. I was impressed, instant to get through, they then put my call directly onto a Company (which was All Clear) who gave me cover no problem, it was for me cheaper to do a year cover that my one off one, as I was in Berlin end of this November.
If you don't have any luck, Google BIBA and ask them.