From what you have told us mate it sounds like the pump itself is working (obviously as the car starts) that goes for all your sensors etc hence no CEL, then after 20 secs or so your car sputters to a halt. What I am suggesting is that there may be another part of your immob which needs a signal after starting the engine within the 20 sec window, obviously if it does not get the signal it shuts down the pump, (I have spoken to sigma sam on your behalf, just waiting on a reply).
In the mean time try starting the car with your ear next to the pump (use a friend/partner to start the car) and see if the pump stops before the engine.
Cheers Iain