Well, well, well. What an interesting day!
I pulled the ECU out to run some tests on it, and what's sitting underneath it? A Powerchip !! Must have been in there when I bought it. No-one ever mentioned it.
After removing that, and putting the ECU back in, I re-ran the KOEO test : 11 - System OK
I could also run the KOER test: System OK.
So, it seems the Powerchip was what was responsible for the 15 & 67 codes.
Now I'm back to what I originally wanted to do : trying to work out if I actually really have a problem with the stalling/idle hunting in hot weather. I've removed the Powerchip and will leave it out for 100Ks (to let the ECU relearn), and see what happens. Then I might put the Powerchip back in and experiment with higher octane petrol. They claim that some of their chips are really tuned to work with that.
Might do a couple of dyno runs in to see if the Powerchip *really* makes any difference at all.
Thx for the help