Yup this battery number nonsence is to enable wumin to change them.
Trewy and I and others stress this all the time, "GET THE BIGGEST YOU CAN AFFORD"
Don't go to Halfords, if you have a costco near you, £60 will buy a kick ass battery.
If you have an agricultural shop near you go there and get a tractor battery (they get quite big and don't fit in the disco battery bay without modification) so measure the bay first and go in armed with sizes.
If you can get a 100Ah, 700 CCA battery you buy it, simple as - biggest is better here.