No expanation- and, oddly, the fans are all still running - but suddenly the throttle does nothing. Cycling the panto and MCB does nothing, nor does a physics reset
Rosenheim. Found the problem, anyway. Apparently I was just a fraction too slow on a Sifa response- not enough for the brakes actually to kick in, but enough to trip off the MCB. And on the 155, the MCB takes the transformer with it- that has to be turned back on, which is the step I was missing.