I have used "the mesh company" from warrington several times and found them helpful and quick to deliver. They offer steel (plastic coated or galvanized) or stainless steel mesh in Nat, Lang & TBH sizes, and sell from packs of 3 upwards.
I have used "bee hive bits" from Bromsgrove for other beekeeping metal work and found them similarly good, but I've not bought their mesh before. They seem to offer just Nat in Steel but sell singles.
Both companies have websites you can find using your favourite search engine.
As an aside, whilst checking that these companies were still trading, I searched for both. Within the results, I was surprised to find a thread on another UK beekeeping forum answering the same question and on 26 Jan 2013 a mod on that forum answered...
suggesting that the OP contact Dave Cushman.