I'm happy to say that after nearly two years (on and off) of pulling my hair out, the engine is finally back to it former self and running as it should. The final piece in the puzzle (lack of midrange) was that even though it was running well, the regulator was still too lean.

I picked up a bottle of coolpower MV purple oil and have run through about four litres of 30% nitro 15% oil. Whatever is going on compression and ignition wise at the moment, you can richen up the regulator and needle with impunity and it's burning it without question, billowing out smoke galore and hauling that plane around the sky like it's attached to a huge bungie cord. The tuning range on the main needle is quiet broad now.

I can't say the engine picked up any significant RPM on the purple oil compared to the blue. Once everything beds in I'll probably drop the oil back to 10% although I don't mind the oil on the bottom of the plane.

Just for comparison I run an APC 18.5x12WPN cut back to 18".
on 20% nitro 10% coolpower blue ~7,250rpm
on 30% nitro 15% coolpower blue ~7,650rpm
on 30% nitro 15% coolpower purple ~7,650rpm

The extra 400rpm on the 30% fuel is very noticeable, and so is the increase in fuel consumption.

Finally back to practicing flying again, rather than practicing take-offs and landings. Thanks to all the posters for their advice and support.