Thanks Amca.
One thing that worries me about "careful" disclosure of medical issues is that unscrupulous insurance companies (are there others?) would disqualify claims if they discovered any pre-existing medical condition even if it had nothing to do with the travel problem. After all, the whole point about taking out medical cover is to protect from exceptional but expensive treatment. A few hundred pounds, even a few thousand, I could cover (with difficulty) - £100,000 hospital fees, no way. So I am not just looking for price but also certainty of pay-out. Nevertheless, I think you are right.