Hi all and many thanks for the suggestions but unfortunately, still no luck.
1. Did the 'listen for the fuel pump running' and yes it did.
2. Checked the battery before cranking at 12.04v. While cranking that dropped to around 10v then returned quickly to 12v once cranking ceased.
One thing I forgot to add to my initial post was that on first repair attempt, the change of the crankshaft position sensor, the car did start but I just let it run in the driveway and after about 5 minutes (I guess as everything stated to warm up) it stopped again and wouldn't restart.
To me it seems the problem is electrically based as with a lack of fuel the engine wouldnt start for sure but on this particular problem, the engine just cut off like someone had thrown a switch. Same when it ran in the driveway, no spluttering or coughing, just instantaneous cut off.
Next step is to check the relays in the RCM again but as of right now I'm still scratching my head on this one