Does that radiator bleed ANYTHING?
Even air?
Make sure you have system pressurised to at least 1bar on the system gauge (by filling loop method) before you even attempt to bleed.
if one side is hot, its the hot feed that's blocked. Is there a knob/valve on the hot end you can turn. It may be jammed, gently tap it or WD40. Bleed when you turn the valve.
I haven't a clue which pipes have been swapped over,I was just asking question for OH who is trying to sort his Dad's radiator out,with out much luck as you can probably tell,that said, he's usually quite good with all things plumbing, he's just not that good at explaining himself. Hes gone to have another look at it, am sure he will get it sorted eventually.Thanks for all advice.
First, ensure that the valves on either side of the cold radiator are definately open. Turn the heating on. Turn off all the other radiators -- There may be one radiator that you can't turn off.
If the cold radiator starts to warm, this indicates that the system requires ' balancing ', which is done by adjusting all radiators until the correct balance is achieved.
It may be just clogged up ,try turning all the other rads off this sometimes clears any gunk/air in the rad...Alternatively take the rad of and flush it through with a hose pipe if lots of gunk comes out the system may want power flushing, not cheap but amazing how much difference this can make on an old system.
Yes ... Turning others off will help force flow through bad one.
I suspect air lock or bad valve. Make sure lockshield valve (opp end to inlet control) open enough.