I've just found a company Vacuum Coatings Ltd based in London who will restore or make new front (first) surface mirrors at very reasonable prices. They are making a new mirror for my Thornton Pickard Ruby reflex which will be 16 ($25) which is very reasonable. I've opted for 2mm glass but they can supply 1.1mm.

They expect it to be ready by Thursday which is a good turnaround.