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I might give that a go if the key is a bust, cheers. I had thought about a socket, but then the 4 sides v 6 sides problem stopped me from trying. But if the key doesn't work, then a little brute force is not going to make things worse. Trying to remember now what my smallest socket is. Got as far as considering filing the outsides of my smallest spanner, but it was too big. Also just remembered I have a drawer of random vintage spanners I nabbed at a car boot sale about 20 years ago that could also be mangled with no love lost.
Seriously, if someone trustworthy said "For £200, I will make a new, not fancy, radiator appear on this wall, plumbed in" I would bite their fucking hand off. But I'm totally fed up trying to find new trades. I'm sure there's any number of handyperson on FB or Checkatrade, but I'm so tired of looking, reading reviews, worrying that they'll fuck something up, letting yet another stranger in my house etc etc
Sometimes tapping a small socket on it can give you the torque and control to release it.