Update.
Having read about purging the engine I decided to do this first.
What I did today was to fit my fuel primer bulb to the metal feed pipe near the cambelt side and also a long clear pipe to the fuel return metal pipe near the cambelt side, put the end of that pipe in a container and pumped away until there was a flow of fuel running into the container. so basicly pumping fuel through the Tandem Pump. While the return plastic pipe was still attached to the metal pipe, and also the feed pipe still attached to the primer pump I cranked the engine and within half a dozen turns she spluttered into life.
I Quickly swapped them back to the original pipes, the ones near the cambelt side, turned the engine over again and she started up no problem.
She's been ticking over now for at least 45 minutes with no hesitation and revs right up to 4000 rpm no problem.
All being well she will start again after switching off and leaving for a few hours.
There's still some tiny bubbles in the clear feed pipe I can see them rushing through when on tickover.
The thing is, it did this before when I first did the pump. It decided not to start just after I put the original black fuel feed pipe back on.