Yep, it's that feedback throttle thing that has been pissing me about.

Anyway, I now seem to have beaten it, even if I don't know what I did(!). Moved the spring on the actuator arm in to the innermost hole (I may have had it the wrong way round above, I had previously moved it to the outermost) and found I needed to wind the spring tension up a bit on the adjuster, and bingo, no load engine just four-stroking and meter showing 240V, put caravan on to it (fridge and battery charger, so probably about 250 watts) engine note deepens with hint of more four-stroking and meter steadies at 235 volts.

On to the next task!