Further testing has shown that this problem can be related to cranking speed. If the battery is 100 % charged then it starts on first revolution of crank, but if continued cranking is attempted to bring on the fault, starting gets less instant, and after about 20-30 tries, about 6-8 revolutions before it starts, initial idle(4-8 seconds) is slightly noisy. But if you continue, the fault then becomes apparent, whilst cranking is still reasonably quick, and starting should still be good, the very rough idle then occurs and if any throttle is applied it gets horrendous and continues until ignition off. Try again, and it usually runs ok!
Battery load test is very good and this vehicle is fitted with a large battery, charging ok, starter current draw is normal, all battery connexions and earths good, approx 20mA quiescent current drain.