http://www.yell.com/s/timber+merchants-tyne+and+wear.html
As mentioned, the closest you'll get to 20mm is 18mm sheets, or 22mm flooring ply (t&g) The latter would be best, see if any of these guys do it, looking about £15ish a sheet of 8x2 (22mm flooring) or about £20ish a sheet of 8x4 (18mm standard)
As mentioned, the closest you'll get to 20mm is 18mm sheets, or 22mm flooring ply (t&g) The latter would be best, see if any of these guys do it, looking about £15ish a sheet of 8x2 (22mm flooring) or about £20ish a sheet of 8x4 (18mm standard)