Dear Prickle,
It is certainly a strange one
My problem, which is the same as yours ( I don,t know whether it is fuel related or air related or spark related ) This is a problem because you need to be 100% sure you are gettting all of these in sufficiant quantities to get the engine to start properly.
My hunch, at the moment ( for what its worth ) is the fuel pressure regulator. What puzzles me is how the car runs fine ( ish ) if you spray something into the inlet manifold. It,s almost as if the engine isn,t get enough fuel.
On a different point, to do with your engine sensors, I have known these to cause all sorts of problems ( from packing up altogether to just packing up intermitently to causing problems with other sensors etc. ) One thing I have learned is that these sensors can look as if nothing is wrong them and appear to be 100% fine. But it is there inability to keep the thing that they are sensing within the min and the max that has been set by the E.C.U. that causes them to go faulty. No amount of cleaning will solve there problem, renewal is the only option. I have also learned that any of these engine sensors can be faulty without bringing on the e.m.l.
It would be nice to the bottom of it
I hope this helps a bit

Vince,