Just a quick report
Starting the app on the speaker setting triggers oscillation, don't do that, start it in headphone setting

In the original constellation (apart from R1, see below) I measure on the FET G=0V,S=0.32 and D=0.78
Today I was getting oscillation on the Amp at any higher gain and/or volume settings. I switched the 4k7 on drain for a 10k which sorted that out and may have stopped a rhythmic artifact that sounded a bit like a cap discharging. I'm a hack, tell me why that worked. Now I have G=0,S=0.34,D=0.99, Oh and I just checked again and the rhythmic thing is still there but it disappears after a minute or two.
There is still some noise and some clicks here and there with the amp. I have this software on my computer and as I remember the amps were quite noisy, but the clicks shouldn't be there. I have 510k now for R1 (PRR's schem) thought that should stop it clipping.
The delay is sounding good, and quiet and the Noise filter works to reduce the overall amp noise.
The iPhone mains power supply makes a huge racket when connected, which is a pain in the balls because I think this app really draws power.
That's it for now, any suggestions, I'm all ears.
Edit: I tried taking the 4k7 out Paul, but sound quality went downhill.