From experience I would suggest actually seeing it done first time round, it is really simple but its best to see it being done as I think you dont quite understand just from words.
If you do get someone they will probably charge u for having to drain the system then bleed all the other rads in the house which is more time consuming, but if u have only one then shouldnt take too long, maybe 30mins for two rads max.
Not sure on price, wouldnt say more then £100 and thats with an expensive plumber.
Just ask around near u, im a diyer and if someone asked to change valves i wouldnt charge more then £30 labour for what ur describing.
Didespitempharsis303.s optimism there are several points you may wish to take on board before you "do it your self".
If its a sealed system the "refill and bleed" procedure can be relatively simple.
With an open vent system the refill and bleed time can vary, I have carried out repairs to two seemingly identical houses only three doors apart on a newish estate up hear in the North West.
Both were open vent.
One took three quarters of an hour whilst the other took over two and a half hours due to an air lock.
Also the time taken to remove the existing (faulty?) valve will depend on how difficult it is to remove, I carry various tools in my van, one of which is a set of 18" stillsons that are some times needed to persuade a reluctant valve to leave the radiator.
Be careful you don't end up pulling the rad off the wall.
Before you embark on your valve changing exercise consider the time of year and the reason for considering a change of valve.
But first the valve, have you considered leaving the valves in the "on" position till the spring?
Is the heating system ok apart from these two valves?
Would it be better value for money to change all of the valves when the system is drained down?
Better than that, how about thermostatic radiator valves which would give automatic temperature control of individual rooms when the central heating is on.
What is the cause of the leaking valve, is it the condition of the system water, are these leaks a fortent of a bigger problem that would be spotted by a bloke or bloke-ett that has expierience of these things.
I am not trying to put you off having a go, thats how I learned most of my skills (aided by making use of the public library,s technical literiture section)
My main concern is that you embark upon a well intentioned adventure at this the busiest time in the heating system repairs calender, which could mean that will mean you have difficulty obtaining proffessional help if things go wrong.
As emphasis303 states, its easier once you have seen it done as you wll expeirience first hand the problems that can crop up. (and they do)
My advice would be to leave the valves fully on till the spring when you can take your time with none of the "my Mothers coming for Christmas Dinner and there,s no heating" remarks from those in doors, why risk a DIY disaster this side of Christmas.
Tim
PS, you may be able to tighten up the valves, but once again, if they do not leak when in the fully on position, why risk it.
PPS, I note that 303 is a man of the Black Economy with no tax, NI stamps to pay, he does not have my overheads to sort out, also he wont have two million quids worth of public liability insurance should things go toes up.
I am sure he/she is quite caperble of doing a competant job but please dont say we are just the same but that I am more expensive, its just that I dont have a seperate day job to subserdise my plumbing and heating work.
