Update to the thread. I know the purists will recommend going to the Wiki but all the links are outdated etc.
However!
I bought and installed a PowerPlus LP85-L yesterday and I think thats it the biggest battery you can fit in the standard compartment at the moment for a low price.
317mm x 175mm x 175mm
85 ah
Wet lead acid
Also cheap as chips. £70
https://www.ebay.co.uk/itm/Campervan-Ba ... 890.l49286
Hopefully this reply will be useful for someone like me who read every thread in the hope that someone would just say. "BUY THIS ONE"
Happy camping!