Ah, so that's it. Do they last a really long time as they are incredibly expensive, the ones I looked at were £85 upwards . . . . you could buy quite a few car batteries for that.
Leisure batteries are designed to be used & run down to virtually nothing but still having the ability to be charged back up without having damage caused to the cells. Car batteries will not stand this kind of use & will soon have damage cause to the internal plates & they will no longer hold charge.
An 85 amp hour leisure battery should be availabvle for around £45-£50 if you look around.