I removed the dashcam, pulled piggy-back fuses, replaced the ones back in their slots, the car started fine with no warning lights. Interestingly, I got a message from the tracker company that the battery had been disconnected. This could be from removing F48 trailer permanent live.
Left it for about an hour, started fine and drove 7-8 miles, switched off and had the passenger door open for about 5 minutes. Locked it, went back to it and none starter.
All the ignition lights came on as normal, foot on the brake and nothing. No attempt to crank, no clicking from the starter motor. I can't remember if it said press foot brake, or if it did at what point.
The epb still engages, the wipers work and the windows go up and down. Both keys tried, no difference but still locks/unlocks.
Obviously battery went flat so called out recovery who put their booster on, and nothing, no attempt at cranking, no click from the starter. During his efforts, the car gave a fart and dropped right down.
The car has been recovered home and is on a C-tek overnight.
I can try to get the codes read tomorrow and that may give me a bit of a clue.
I was optimistically hoping I may have drained the battery while messing about inside the car, but I'm less hopeful ATM. I'll see if I can get a socket and bar on the crank, but I'm struggling to see how it would seize after it was turned off.
I'm hoping it may be a faulty brake light switch, but am open to any suggestions.

Ok guys, to round this off, it was the engine earth strap.
When I was checking the earths, I assumed the engine earth was OK and was checking the body earths by taking a cable (jump lead actually) from the negative battery terminal to the top of the engine. What I should have then done was earth the engine to the chassis, it would have then started and given me the answer. Just goes to show, never assume anything!
I hope this helps someone in the future.
I can only say that the indi I use was brilliant and fitted me in, diagnosed, and sorted it today, which was important as I'm PX'ing tomorrow.
So, indi one, insurance company nil (which is another story).

Thanks for reporting the fix far to many posts on here without a resolution

Yeah, that is frustrating. If nothing else, when someone asks a question, someone else may just remember they read a post and link it.
In fairness, I would have to had taken it to the garage anyway but it would have been less stressful having diagnosed it myself.