I once owned an Opemus 6 bought in Finchley north of London. The transformer was a custom made 220v/12v toroidal one able to feed a 12V 100W halogen bulb. Not rocket science.... The only difficult part was the box into which it had been put because it used riveted covers instead of screws.... Maybe to obey safety regulations regarding user not able to open the box.
Anyway, dismount the switch, go to the nearest electrician shop and ask for a replacement. It should not cost you an arm and a leg. Only a leg ;-)