I don't know if all of this is important but, knowing me, if I leave anything out it will be the most important bit.
I got in my car which is a 1995 XJ6 (x300) and drove it for 12 miles. Not using any lights at all. No heater, no radio, zilch. The battery indicator was showing halfway between the 12 o'clock position and the red line.
I parked up, took some kit out of the boot and locked up and set the alarm with the key fob.
3 hours later I went back to the car and unlocked it using the alarm fob. Before opening the car door, I opened the boot to put some gear in there (all the boot carpets and stiff cardboard type partitioning stuff is in there)
I got in the car, put the key in the ignition and turned it. No lights came on and when I turned it a bit more there was no noise from the starter. In fact, it was as if the battery had been removed.
I got out of the car to leave it where it was overnight and went to lock it and set the alarm with the key fob. No joy. The doors would not lock using the fob.
At this stage, I thought maybe I had disturbed something when putting the kit in the boot. The "Kit" is a walking frame like a Zimmer but with wheels. So I opened the boot using the key and checked that no wires were showing anywhere. I closed the boot and gave it a half-hearted try with the fob and it locked up and set the alarm. Just in case, I tried unlocking with the fob and it did the job. I got in and put the key in the ignition and gave the key a half turn and all the warning lights came on as normal and she fired up first go.
All I can think of is that either there is a fault or it's some kind of Jaguar safety device that when you unlock the doors, you can't open the boot first but that's just silly.
Any ideas?
Cheers,
Steve.

 

First thoughts would be the wiring harness at the right boot hinge which can become broken through opening and closing the boot x amount of times in the cars lifetime. I believe that would cause problems with the imobiliser.
Second thought would be the valet switch got rocked but as I know practically nothing about that piece of kit best wait til others chime in.
Thirdly, as a matter of course, try to go over all the earth points and clean them up (with the battery disconnected).
Certainly sounds an electrical gremlin.

 

Something that I forgot to mention is that when I was sat in the car with a totally dead electrical system, at least as far as starting it was concerned, for some unknown reason I took it into my head to try to move the auto gearbox selector. I was 99% certain that it would be stuck in the Park (nearest the radio) position. Now, maybe I should explain that our other car is also an automatic and you can't move the stick when the car is stationary without putting your foot on the brake pedal. Now, I can't say with complete certainty whether I have been doing that out of habit when moving the stick or not. All I can say is that when sat there on this occasion the gear lever moved freely up and down the right hand slot, across the bridge and then up and down the left-hand side. And that was with no key in the ignition.

 

I can't think of any reason why you should have been able to move the selector with (apparently) no electrics - unless the locking mecanism under the little torx shaped screw at the top left of the J Gate was pushed down...or...the car thought the foot was on the brake - might suggest a stuck brake pedal switch. As you said the car had no electrics working during the fault, I'll assume the brake lights were not on.

 

Wouldn't you know that now, thankfully, I can't re-create the symptoms to answer any questions. All I can say is that it now works as it should and that with no key in the ignition, the gear lever won't move at all.

**Is stumped** but thanks for trying Straight 6 guy.

 

The car won't start if it is not all the way in park, or neutral. The neutral safety switch makes sure of this. Possibly, perhaps, it wasn't all the way in park and or the switch is slightly out of adjustment.

I went for a month needing to start my car in neutral because the switch was broken. In park it wouldn't even click or try. The little red P wouldn't even light up. Then I replaced the box to the right of the gear selector and all was well.

 

JackJ .... That makes an awful lot of sense and certainly worth checking if I get the same symptoms again.
Cheers,
Steve