Curiouser and curiouser - I've spent the last half an hour or so swapping transistors and got at least one surprising (and IMHO) cool result
There are three transistors in the OK layout - Q1 and Q2 are listed as MPSA18 and Q3 as 2N2907.
I started off swapping Q1 for pretty much every different transistor I had and it sounded fairly similar with a lot of them, and I still had the problem that it only really worked at all with the input control (B100k sweeping between a 100nF and a 10nF capacitor) all the way up (100nF). This was also true when the socket was empty . . .
So next up I swapped Q2 - none I tried were of any use until I stuck in an MPSA13 - BINGO!
It still doesn't work as it should
but it does work as a sort of fuzzy square wave tremolo which starts off quite quickly and slows down as the note decays - I like it !
The input control also seems to have an effect across its whole sweep, which is the first time that's happened
One thing which seems to be helping it do it is a rogue MPSA18 which has an hFE of around 650 compared to all the others I have which are around 430, I tried a different MPSA18 and the effect was much less pronounced.
So I still don't have a working OK, but I do have a working circuit which is making interesting noises, so I'm going to consider that done and I'll put it in an enclosure when the Parallel Universe is finished. I've not yet decided on a name, but OK-ISH seems appropriate.