After having been away for over a month, I returned to the car and found the battery was dead, no alarm chirps or mileage on the dash I suspect the drain was due to me leaving the bluetooth OBD reader connected, but can't be sure (yet).
I opened the car with the key and got into the trunk, connected booster cables to a truck, then tried to start it, but nothing, The dash lights came on and went off as expected, but no crank
The lamp light came on once as did the transmission light. I checked the gear shifter LED which was on in (P) park. I suspected I may have blown a fuse or relay, but everything looked fine, even after swapping in a good relay for the ignition.

After some head scratching a cup of coffee and trawling the forum, I remembered a post discussing the SCLM, which could have failed.Then I realised the problem.
I had opened the car with the key when there was no power, then tried starting with booster cables, but got nothing but the usual ping noises and dashboard light process.
I needed to unlock the car with the security system powered up. Since I need the booster cables connected, I used a screwdriver to trick the trunk into thinking it was closed then cycled through locking and uncocking a few times. Then when I tried starting the car fired straight up, and after a few runs looks to be back to normal and the battery is holding a good charge.

So, a lesson learned about making sure the SCLM knows the car is unlocked before trying to start. Also, remove the OBD reader if leaving the car for some time.
I feel a bit dumb for not realising it sooner, but we sometimes overlook the obvious.

 

"cycled through locking and uncocking a few times."
uncocking your car... sounds sus.... lol....

i would say it was as simple as the battery being totally dead requires more than just the connection.
i have jumpstarted many cars over the years and on occasion had to leave the cables connected for a couple mins to let some charge get in as the batteries were just so dead they would not crank the car.
on that note tho, i have always had the jag start fairly easily, but recently my battery was going to **** not holding charge. so i replaced it.
and now since putting the new battery it has started way quicker than before. it barely even cranks before it is started.
so i would say my battery was always sub par since owning the car,
and also that it does require some good power to get a nice start so logically i assume a fully dead battery would need a few minutes of connection before cranking.