In reply to wykealpha:
Worth asking your bank if you can extend the cover, but my experience of trying to extend cover on a "standard" insurance policy to cover Nepal trekking is the same as Gld73's - you won't be able to get hold of anyone who actually understands the question, let alone be able to sort the cover and give you a premium.
In the end, I went with BMC "Alpine & Ski", which covers most trekking peaks in Nepal and covers you for over 5,000m (we did the Annapurna circuit, which tops out at 5,416m).
Saying "Nepal" will, however, hike the premium significantly - I understand it's due to the level of rescues, but that's going off at a tangent.
Post edited at 15:23