I would change the o ring but also if you take the blue and red covers off on the end of the valve,then take out white ceramic bits you will then see two X shaped ceramic pieces.Remember how it all came apart and replace the X pieces.They are on ebay for a couple of ££££s.
Or buy new valves for...
Sometimes you can get to the valve without taking the heads off. Open both taps fully,unscrew shroud and you should see the nut that you need to undo. A thin spanner might get in there ?
I used to do a lot of gas safety for councils back in the day and vented ridge tiles often caused problems and especialy with that type of fire.
Do what cross thread said above and swap it for electric.
As above but my experience of twin entry valves are that they are not very good.
Are you capable of removing them and checking the length of copper inside the rad ?.
They often get blocked with sludge.
Yes and yes.
Why no use two isolating valves and then you can easily re-connect the radiator and will just need to pressurise the boiler after bleeding.
It will probably be a ceramic cartridge and not a washer inside that tap.
have a look at ......
New Valve Tap Replacement Ceramic Disc & Silicon Washer Gasket Insert 1/2'' UK on ebay
it is quite a simple job
I have a soakaway that is under my front garden about 6ft from the house.It was there when i bought the house and i have no reason to see it but i know that it works well.
My downpipe runs underground for the 6ft distance.