B3 V6 non starter

[redhunter]

Hi, we purchased a 1998 (R reg) 2.8 Passat that stood idle for at least one year, fitted a new battery and it ran.
We had it MOT,d, it ran for 10 miles ok, now it won't start. When you turn the key, it runs for approx' 2 seconds and stops! The fuel system has been checked for blockages etc, the crank sensor has been changed but it still persists.
Could the problem be the ECU,or the immobilizer? What indicators are there to show these faults? All suggestions are appreciated!