Hi bunnyarmy, In that case you have three options. Turn off the radiator valves - both ends - system off.
Either undo the nut completely, wrap ptfe tape round the olive and threaded end and refit the nut. You will get water pissing out of the joint while doing this. The major problem may be that the joint is defective and you will get flood.
Safer option is to hire a pipe freezing kit, freeze the pipe and carry out the repair knowing you will have 5-10 minutes to complete it.
Thanks everyone! I rang my builder that did my renovation work and he told me where to drain the system in the cellar. Just needed a new compression reducing internal coupler instead of an olive as the pipes are small. It was pretty simple in the end and at least I know where to drain it from now for future!
I had the same problem I had a regular boiler so I put a bung in the header tank released the nut then ptf. If you fill a straw with water but keep your finger on top the straw the water doesn't flow from the straw in needs air behind it. So I thought if it works for a straw got to work for a long copper pipe and low and be hold. I'm not saying you should do this but it worked for me . Good luck
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