Murmansk wrote

I'm trying to dismantle one of those outdoor swings - like a wooden sofa
suspended on chains from a wooden framework with A shaped supports at
either end.

The bits of timber are attached with what I think are called coach bolts -
a domed head with a square bit underneath.

Yes, thats a coach bolt. Originally used on real coaches,

The problem is that the bolts are recessed down a hole. I can turn the nut
with a socket set but because it's a all a bit rusty and seized up the
head of the bolt turns as well and I can't get hold of it to stop it
because the head is domed and down a hole.

Any ideas how I can get hold of the head to stop it turning?

You can drill a hole into the dome, into the square knob thing under
the dome and put a thing a bit like a thread tap, but tapered, forget
the name, into that hole and put a spanner on the square end of
the thing I cant think of the name of.