I think I fixed it! I thought I'd put down my diagnostic procedure - maybe it will help others: 1) Thought it was an immobaliser fault, but in ignition, the LED on the door glowed steadily for 3-4 seconds then switched off, indicating that the immobaliser was not stopping the engine starting 2) Many suggested it was something to do with the fuel pump. I checked the diaghram in the fuel pressure regulator by removing one of the vacuum hoses and smelling and looking for fuel. nothing. I finally removed the possibility of a fuel pump problem when the problem progressed and the engine started just cutting out at idle during normal use, rather than failing to start. The engine cut out cleanly and immediately, which seemed to suggest I did not have a fuel problem. To check this, I removed the fuel pump fuse (F18) while the car was running, and rather than die immediately, it coughed for about 20 seconds, desparately sucking fuel from the tank. Hence I convinced myself it wasn't a fuel problem. 3) In desparation I took the car to hendies, who said it was a camshaft position sensor problem after putting on their diagnostic. I changed the camshaft position sensor - no difference. 4) I bought a OBDII lead from ebay and diagnosed the engine myself with Vag-com. I first cleared all the previous errors, then ran the car until it failed, and diagnosed again. The only error was intermittent crankshaft position sensor, wich makes sense for the problem I'm having 5) Installed a new crankshaft position sensor, and with luck, it now works!! Thanks to everyone for all your help!!