Like Marcel with his compressor pedal, I came at this again this morning with fresh eyes and immediately spotted the problem - the leg of the transistor which connects to the gain pot had broken off. In the confines of the enclosure it was impossible to tell, but on close inspection it was pretty obvious, and as soon as I wiggled the wire it popped right out.

So, emergency trip to Jaycar, bought a new MPSA13, soldered it in, and the fuzz is back!