This is going to be absolutely no help to you Sylvia, but I just wanted to say I empathise. About 15 years ago we changed our medical insurers (because as we're in the US we have to be insured) and right at the end of the process they suddenly decided they wouldn't cover me because I was on anti-depressants. I got nowhere with them, even making the point you made: that surely a treated depressive was less likely than an untreated depressive to get ill, and furthermore, since I was being treated, I was in regular contact with doctors so that things were likely to be picked up at an earlier (and cheaper) stage. All to no avail. However, my doctor went incandescent on them, and pointed out how discriminatory this was, and they caved. As I said, no help to you, but this is a worldwide phenomena.
Love
Jennifer
PS. Apparently Which publishes a review of holiday insurance in March which highlights pre-existing condition coverage. You should be able to find it at a local library. Also Age Concern offer travel insurance.