In reply to CPH:
It does seem extraordinarily complicated, compared to changing a small button battery every couple of years. "Ker-ching!" for the main dealers though I suppose. I guess if I were to look for a less cynical explanation, making the key a sealed unit does mean it can be properly waterproof at least.
This reminds me - I noticed the other day that the 'plip' on my van key doesn't seem to have quite the range it should, probably time to change the battery in that.
Edit to add:
Bah. Later. It wants a CR1220. I have half a dozen 2032's kicking about and a blister-pack of LR44's with one missing but none of them. I think this must be the first time it's needed a new battery since I bought the van 4 years ago
Post edited at 10:14