Thanks for that link, Andy.
I didn't get any of those, but did
this one which is costing me £20 in total (for 5).
The ones you linked to would have been okay, but very expensive at over £135. I did find some today of the right size (18x18mm) and colour - better than the ones I have now - and pretty cheap, too. I may buy them and retro-fit them in future, or do a completely new panel from scratch. I'll see how useful I find this one when it's finished.
http://uk.rs-online.com/web/p/push-button-switches/1192942/Though I wouldn't buy them from RS.