Simon H wrote:
If it's a brass plug, then heating it is perhaps not the best option,
brass has a tendancy to melt.

The best method I can see is to file the nut down so it has two very
defnite sides on it, which you can get a big spanner onto.

Alex

The first thing I would try would be to see if it's possible to 'persuade'
a six wall socket onto the rounded nut, maybe after a bit of judicious
filing. Then I'd try what Alex suggested and attempt to file a pair of
flats. If that didn't work I would lose patience with the job and find a
big nut that fits over/around the offending plug and weld it on. Well, I
would if it was a steel plug anyway.

Simon H
Thanks for the advice, there are a couple of reasonably flat sides I have
tried an adjustable spanner on, but I couldn't get enough leverage without
the spanner slipping, so I thought I would try some stillsons to get a good
grip with, if not I'll try filing it as you suggest, and use a conventional
spanner or socket.
Thanks
Shane