Hey daveydog (if that's your real name

),
The button unscrews and there is a small spring inside that seems to locate a bulb - the bulb just pulls out.
I've attached photos - I hope the detail is OK.
The bulb on the left is the standard 12V 1.2W 'bayonet' style that is used in the advisory lights, indicator lights, low fuel, alternator etc. whereas the smaller one on the right is also 12V 1.2W but is a 'pea' type and just pulls out. As far as I know, the only place it's used in these cars is in this switch.
Regards,
Chris