On Thu, 20 Sep 2018 16:03:03 +0100, John Rumm
wrote:

snip

If its easy to add into the ring, then that is slightly preferable,

Understood (and I would try very hard to make it so).

but
a spur is also acceptable.

Ok.

[1] I think it's all plasterboard on battens but as yet have no idea
how much room there is behind the location of the additional box (or
where the battens are etc).

You can use a stud finder,

Yup, I have one at the ready. ;-)

or a metal detector, or even just a strong
magnet to find the fixings in the PB.

Understood.

Ironically I did see the wall in question as it was being done (re
remembering where the battens / studs were) but that was 2+ years ago.
;-)

Cheers, T i m