I can highly recommend simple travel insurance. I found them through the which consumer magazine as they are good for covering medical conditions. They even covered me following an operation 6 weeks prior to taking out the cover.
We get annual cover and for 2 adults and 2 children (and the children can be up to age 22!) the total premium is just over £47.00 for the year - you can pay a bit extra to waive the excess if you wish and there is an additional premium if your over 65, or also, if you wish to participate in various sporting activities.
I've not had to use the company however so cannot say how good they are on a claim!
Before taking out a policy, its always best to ring the medical number given to check if your medical conditions are covered before taking out the policy.
Hope this helps