Your choice on a mains transformer that matches what Gaugemaster specify for this controller, 1.1A, 18V AC. RS components, Farnell, etc.. Or simply buy the Gaugemaster piece, which has their warranty as compatible with the Combi as it has a cut out for over current. (I'd go for the latter to definitely protect the Combi.)...Do I need a manufacturer's transformers or where else can I buy them? All my other PSUs are Hornby and seem to trip out annoyingly when I am testing locos that I am trying to fault find. I guess output currents of the controller will be important as well as input? Any pointers appreciated.
The 12V 1A output to track of the Combi will be plenty for OO and HO. Most current products with can motors will draw 100 to 300mA if they are in good order. Older product, H-D and similar with open frame motors, and Heljan diesels with larger Buhler can motors, are likely to draw 500 to 800mA. If these latter have a drive train gummed up with dried lubricant after a long time on the shelf, they may trip the Combi: that usefully tells you to sort that out immediately, so the motor can turn freely before resuming testing. If the drive train is turning freely and an open frame motor swiftly gets hot, that's usually a weak magnet that needs remagnetising.