Flat battery symptoms

I have a P38 diesel that has given me a bit of a headache with the electrics - in particular dischaging of the battery.
I have had a new battery since May this year as I was sick of the old one going falt but this week the new one is flat after not using the car for 2 weeks.
I've tried jump starting it but still the engine won't turn over (just tries to then all the lights go out on the dashboard - just like it's got a flat battery!)
I've tried using the battery from my L200, but the symptoms are the same.
I've put on a new starter & solenoid but the symptoms are the same.
I've tried disconnecting the alternator, but the symptoms of a flat battery are just the same.
Next step is a box of matches & a gallon of unleaded unless anyone out there can give me a couple of pointers.
Cheers

Need to wire either an ammeter (10 A range) or multimeter in series with the positive lead on the battery and see how much current drain is present after the Becm has gone to sleep (approx 5 mins after locking the car). If you still have a high drain, it's a case of pulling fuses etc. until you identify where it's coming from. You may have to disconnect the alternator also in order to check that. It's a case of trial and error I'm afraid.

or clamp meter