Michael Chare wrote:
I want to tighten a chrome plated hexagonal nut on a central heating
radiator valve without marking it in anyway.
(Part of a so called designer fitting.)

Any suggestions as to the best ways of doing this? I was considering
getting a spanner of the right size and if possible using some thin
paper between the spanner and the fitting.

Oversized open ended spanner with two small pieces of plywood in the jaws.

Why do you not want to mark it?