Had a similar problem with a Citroen a few years ago. a quick word with a local none franchised outfit,
the answer.
Try a tow start.
Hung the thing on the back of the Disco, and it started easily, and has continued to do so ever since.
Never did figure out what the original problem was, but from what I was told, it's not unusual, and the tow start thing often clears it.
Pete