Alternator. I had a drain on my old D3 for ages, couldn't find it. Alternator was putting out over 14v so thought it was fine. Eventually, I worked out that the alternator diode pack had failed and it was putting out an AC voltage, as well aa the DC.

With the engine running, measure battery voltage using AC setting on multimeter. Should be zero. Mine was about 0.75v AC I think. New alternator sorted it.

Just purchased one of these.

(link wont work!)

But its this on ebay Imazing Portable Car Jump Starter - 1500A Peak 12000mAH

Me too!

We were packing up the caravan and I was in and out the car, closing the doors because it was raining! So when it came to driving away, car wouldn’t start!
Had to get the warden with his battery pack!Quick jump and we were gone!
So after reading this thread, I’ve bought Imazing too! 2014 vogue se

Me too with the packing caravan routine, come to depart, and flat battery.
If I start my ff, and don't let it run for a while, battery flat
I've got into the habit of turning off the interior lights every time I open the doors if I'm loading the caravan, bit of a pain, but I'm sure it helps.

Wogger, if you hold the interior light button for a few seconds it will disable the interior lights permanently! Until you hold it again to re-initialise them...

Easier than pressing the darn button every time you open a door sir

I have this Startzilla 2012 from Telwin. A bit more expensive but very well made.

 2011 RR 5.0 SC & 2021 New Defender D300

Cheers Craig, you live and learn, had the motor for nearly five years and didn't realise

Me neither!
Could have saved me a couple of shillings

Well done CTC  2014 vogue se