I've finally got fed up enough with a cold water tap in a toilet not
working that I think I might fix it. (In my defence it's not utterly
trivial as I'm going to have to freeze the pipe and insert a valve so
it's an easier job in future.)

The fix is probably dead simple (a washer squished into the wrong place
I suspect) but I can't immediately figure out how to dismantle the tap.
The taps look a bit like this:

http://www.screwfix.com/prods/11604/...ps-Chrome-Pair

I'd

have thought that the metal top (inside the red/blue ring) would screw
off with a little downwards pressure, but it doesn't. What's the trick?

As an aside, any recommendations on what full bore valve to use (we
have little enough pressure that I don't want to use a non-full bore
one). I see these from screwfix, but it seems a little pricey:

http://www.screwfix.com/prods/21860/...alve-15mm-Blue

Thanks,
Piers