Andrew was thinking very hard :
I could leave my 1998 Astra F estate (petrol), with a factory
immobiliser (that shoed a slow blinking red led on the dash)
for 7 weeks and it would start without an issue.

My current Astra H (59 reg) handbook says disconnect battery
if vehicle is to be lft for more than 3 weeks unused. Seems
a bit OTT to me.

Modern cars have much more electronics systems which all draw some
current when off, much more regular use is made of cars these days, so
not usually a problem.

I don't make such use of my car, so it sits in my garage whilst unused
and I have it plugged into a charger. The charger comes on for 20
minute each day, timed by a Smart Plug, just to keep it near a full
charge.