I have hopefully, made some progress on fault-finding, an auto-electrician has pointed the finger at the EGR valve, which would fit with the problem happening, once warmed up, and you go into overrun, the valve should close, if it does not, you will have issues with fuel/air mixture, it is an intermittent issue, which makes diagnosis tricky. just odd every single dashboard light seems to glow, when you do, along with the big red warning exclamation mark.
I replaced the EGR 3 years ago as a precaution, and it wasn't dirty when we took it off now, but maybe the solenoid is dodgy. Will replace the valve, and see, I hope it is the issue, as the car runs perfectly otherwise.