Thanks very much, gents!
The SCR and diode spec's make sense, partiularly the "isolated tab" parts - can't bear to think of what'd happen with the heatsink/bracket shorting the two SCR's....
The mill power feed's a ready-built unit sold for one of the Chinese small mills (OK, it's actually three of 'em rescued from a skip...) - it has a 90V DC motor in it, and a tendency for the control board to burst into flames... Fortunately I have a known-good board to work from! I have to admit I'm a bit worried by the 90V motor being fed from the 240V mains, strikes me that could be why the controllers aren't too reliable...
Dave H. (the other one)