Finally sorted it!
Seems I had two bad LM393s in the last batch I bought. What are the chances of swapping one out, suspecting it to possibly be faulty, only to replace it with another brand new one that was also faulty!
Grrrr, dodgy Ebay source I suspect.
Thanks for the advice Ken. Onwards: there's another 4-stage sequencer to build now, along with trying to transplant two Barton Multi-Window Comparators into this project!
Neil