Thanks for your replies.
I checked the T + C of both holiday booking and insurance before contacting holiday company as I'm a bit stuck.
The insurance company needs medical confirmation of "unfit to travel" which my GP can't supply at the moment as my condition is undiagnosed and I'm currently undergoing tests, although I know that I'm not fit to ski. The situation is complicated by the fact that I went to my GP early December with this problem, it was mis-diagnosed (I was aware of potential problem re skiing) so an entire month was lost in trying to sort the illness out. I don't have private medical cover and there isn't enough time to get the tests done, check the results, possibly consultant follow up AND cancel / amend the holiday booking.
I approached the holiday company, explained the circs and asked what they'd suggest. It was their suggestion to move the booking back in this way. They have charged me an admin fee for doing so, which I've happily paid - I've paid everything that they've asked for and we've had to downgrade our rooms as the ones we had originally booked are apparently not available (even though the chalet was showing as empty when I contacted them).
The holiday T + C assume that for an amendment, it will cost more, so there's mention of paying additional amounts, nothing about refunds. It is an unusual set of circs, but it seems to me that if I've paid everything that they've asked for, in full, then they should refund the difference.
I'd have taken it in credit against a holiday next year if they'd offered, but to just hold it, for no apparent reason. . .