Sadly wrote:

I have a stud wall in my house that is 85mm uprights with 20mm lathe
and plaster each side.

It's lath and plaster. A lathe is a machine tool...

I have access to the inside of this wall (at the top and bottom) at one
place along the wall, and would like to run cables down it for a ring
main. The regs say that cables must be at least 50mm deep if they are
unprotected. There is an area 25mm wide that is 50mm from each side of
the wall.

So, should I run my cables in this wall?

The problem here is that with access at only those three points you
couldn't fix the cable well enough to constrain it to that middle inch.
If you're prepared to knock in two more holes and clip at five points
it might just be OK.

Would I be better off putting them close to one of the sides of the
wall and adding a socket in the same area (to turn it into a permidded
zone)?

That's certainly an option, and then it wouldn't matter where the cable
went within the thickness of the wall (assuming the switch or socket
location can be determined from both sides of the wall) - but you'd
still need to constrain it to a accurate horizontal or vertical run to
the electrical fitting.

Comments please...

Does that help?
--
Andy