Duk a la Technical......Have a google on site such as Tanya batteries or one of the others. Just google leisure batteries.
You will need the physical dimensions, Ampere hour capacity and whether the current battery is gel or wet type as they have different charging regimes. Also whether your current battery is sealed or not. With this information you can choose. Banner batteries have a good name, as do Varta, but there are some good cheaper options out there; horses for courses, really
I think that we had better bring this back on topic or poor Hancock will think that we are all very deranged people.........
Seriously - the above is what I did when getting a new leisure battery for the last van. Ended up with an Alphaline sealed one- next day delivery. Don't know how good it was as sold the van some 3 months afterwards. Beware of getting one that is too tall - I had to get a new battery carrier as it was too tall by about 5mm!!