can these valves be re o ringed ?
Yes - that's what people have been discussing.
does the system need a drain down?
No.
how does the water not escape?
Because the valve closes when you remove the headwork.
are the original colours still visable after being in crappy water for...
Unless some monkey smeared jointing paste, or plumber's mait, all over the threads, you'll only have to get the nut moving a bit and then you can spin it off by hand. A tap with a screwdriver and a hammer will often do it.
The Belmont is a marvellous valve - well worth maintaining rather replacing, especially since most Belmont replacements will need a pipework modification.
So it's £6 per rad for another 10 years. So what?
Let's say that you can find them for 50p each - can you justify the time that you...
Whether you need an open vent or can make do with an AAV depends on your existing venting arrangements, so please provide more information.
And which "regs" are you referring to?