Well I have booked my car into a local Ford dealer, although they couldn't get me in until next Wednesday

....I took the car for a short drive today, and the engine warning light has come on now, solid yellow.
I've tried to do a bit of diagnosis myself, but I'm no mechanic, I can hear a hissing sound from the rear right side of the engine when it is running, and when I turn it off I hear a hissing sound for about a minute towards the rear left of the engine for a while (bear in mind this engine is horizontally mounted). I think perhaps a hose or something is split, I have checked the ones I can get at, as best I can, I can't find any splits or cracks. My other thought is that it could be a seal or gasket around the throttle body area or something. As for the hissing after I switch the engine off for a while, I don't know, that's what made me think a hose/pipe maybe. Not sure, waiting till next Wednesday is annoying though, I think I'll ring around a bit more. if it actually stops on me before then, I'll be stuck.