OK - nothing else is jumping out at me and troubleshooting boards like this isn't my strong point so we'll just replace the transistor and see what happens.
There's one thing - while removing the RA connector from the board when I did the Rowan mod, I may have damaged one or more of the soldered connections. I managed to pull the entire connector from the board this time (replacing the bearings with SKF) and resoldered it. Whether or not those connections were dodgy after my first intrusion I have no idea - likewise the possible consequenses.
The mount was working just fine up til the Meridian Shift, load was nothing heavy (WO GT81) and there were no cable snags.