Standard batteries and battery positioning help please

Hello all!
I have a 1986 260 Spanish built mk model, Its a 3.3 turbo diesel.
The bugger has been of late giving me starting problems as its now super cold here in Italy. In summer it was just fine.
I have no experience on the Patrol and have started researching the forum. My Patrol is totally standard with hardly any electrical items either installed or required.

The two batteries are one for cranking? The other? The positions are which under the bonnet?
What Ah should they be? I need to change the older battery as it doesn't charge very well and believe this is my starting problem causer. Thus i dont want to buy a new one if its under rated. What were the standard ratings fitted to the Patrol. The batteries do look a bit small in the trays...
Can i have two different Ah rated putting one bigger one for cranking? Positions to be confirmed?
Thanks for any advice, Have a great day.
JM