Thanks for the warning Scottie. I guess the battery must be c. 40Ah which is on the small side but not unusually so. As to being underrated, maybe the issue is more the rate at which the battery discharges while the car is parked, or possibly even the size of the alternator?
Batteries are of course heavy - even a 40Ah one is about 12kg. Weight was presumably the reason for the puny Panasonic batteries in the older models.
Fortunately I have a CTEK that I used to (had to) connect to my Mk2 so I’ll make sure I use it over the winter when the car will be much less used.