Awesome, I've got my very own FAO thread

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Assuming that it is the later style key (with the metal surround rather than the all plastic one):
You'll need a small, flat blade screwdriver. You then want to use the blade to prize the area either side of the plastic bit that holds the key ring, as per pic:
You don't need to muller it, but a bit of force is required. The back part of the key with the wings should be released.
Try and keep the plastic ring that connects the key ring to the key itself actually in the key. It's not the end of the world if it comes out, but it's fiddly to get back in sitting right if the key is worn.
When refitting "hook" the top bit of the cover into the key then position the bottom part to clear the little plastic "ring". Once you're happy apply a little bit of force and it should click into place
The battery size will be a CR2032.
I've made it sound more difficult than it actually is.
Edit: Mark's stolen my thunder

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