Thanks for the advice along the way everyone. The GQ TB42e lives again -the fuel pressure regulator was the cause of the fuel prob.

So to recap, terrible starting and running on gas and even worse running on petrol was caused by a fuel pressure regulator on the fuel rail developing a fault and tipping neat petrol into the inlet manifold. This meant that on gas, the engine was getting gas AND petrol at the same time.

The clue was black sooty crap coming out of the exhaust and also coating the plugs. Disabling petrol delivery meant we were good to go on gas, isolating the problem to the petrol side of things.

Terrible running on petrol was because of poor pressure in the rail (cos the regulator was letting the fuel go elsewhere) and the injectors were clagged up with E10 fuel crap.

Lessons learned include: pay attention to the obvious - loads of black soot and the stink of unburnt petrol. I should have started there. I got sidetracked cos the regulator is basically brand new and I just didn't think about that being involved. just proves new parts can also fail quickly.