i just used brute force (well a little) as you can seperate it using your fingers once you remove the manual key,After seeing @bluecar1's recommendation in the other thread about changing my fob's battery, I thought I should probably learn how to do so.
I wondered if it might be possible to open the fob without a screwdriver but with something reasonably blunt instead. I'm not the most co-ordinated of people, and there's a non-zero risk that I'd gouge either myself or some delicate electronic component if the sharp screwdriver slips.
Turns out it's possible to open the fob using the built-in key. Just put the tip of the key where the tutorials indicate to put the screwdriver, and either (gently) twist or lever the case open.