Just as an update I've made up the dimmer. Used a Knightsbridge polished chrome 2 gang plate from TLC. Which I've used before to make 'specials' like say for AV.
I think the last one I used was chrome on brass. So not that difficult to drill/work. This one - not in the usual Knightsbridge cardboard box seems to be polished stainless steel. Looks just fine, but a nightmare to drill. Blunted a couple of new 2mm HSS drills while attempting the first pilot hole. Eventually used a tungsten one. And broke it on the forth. Enlarged it 0.5mm at a time - it did get easier with the larger sizes.