sorry it would be the main burner then, if all other rads off then cold one heats up
Right then in that case it shows that its only a balancing issue!
Turn them all off ( apart from the naughty rad ) and then turn them on by JUST 1/4 turn!
The "JUST on" setting is very critical. Rather than a small fraction of a turn its best identified as when heat is starting to flow into the rad.
That cannot be set very quickly and can take 2-3 minutes per rad as you have to make a very small adjustment and see the effect a minute later.
If the main burner is only lighting for a small fraction of the time then it indicates that the boiler may be range rated to high and delivering too much power to the heating circuit.
Your boiler is very powerful to give a good hot water flow rate but its totally overpowered for most normal homes. Even the minimum setting will still cause it to switch on/off in most homes.
thanks it seems to be working again and am just monotiring it at the moment
Tony