IT RUNS

So, after a misty start to the day, it turned out quite nice so it allowed me to get back out to the Bus and continue my hunt/saga!
I finished off checking my fuses and relays (Steve above gives a link to one of the many threads I found that all talked about the PCM and communication issues)
The battery had been disconnected for over a day so touched the leads together once more and connected everything back up.
Tried to start it, no crank and flashing security light

Started to search for PCM, pulling lower dash panels and other things.
Found wires everywhere from the side cameras so will have to tidy all that up!!
Checked more fuses in the inside fuse box which is conveniently located right up in the corner of the foot well

Couldn't find PCM and was going to start taking the steering column binnacle off so I could see if I could find the transmitter ring for the ignition key.
Then I suddenly remembered what youracman said about the sequence of events.
So, normally when I use the jacks, I start the engine but I obviously couldn't do that

But it also needs the parking brake on, so I pushed down on the parking brake pedal and put the key in the ignition so I could at least turn it to the run position.............
....NO flashing security light, everything looked normal (forgot to mention that you lost the mileage when it didn't start, replaced by dashes!)
Okay, I have tools and everything in and around the drivers position but I went for a start.........
....and it burst into life, first turn

So, now I'm asking myself, is the parking brake mechanism rubbing on/moving something and every now and then causing this to happen??
I don't know but will take a good look tomorrow just in case
Thanks for the input everyone. There's no real answer but will update if it happens again or if I find anything