Tips to release tight radiator nut?

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Trying to release a radiator nut to remove the radiator for replaster

I have easily removed the nut on the thermostat side.

But I cannot loosen the other.
It's requing so much force that it's bending the pipe work and I'm concerned applying any more.

Any ideas? I'm not sure wd40 will do anything.

The culprit is below

 

[413x]

Blowtorch?
Pretty sure seen a plumber do that in extreme cases

Not even sure I'd want to try it if I had one!