As a matter of curiosity I still have the original battery at nearly 11 years old. I keep it on a battery minder charger that I actually ordered with the car when it is, or will be, in the garage for more than a week. Started today without any issues.
We just had to replace the battery in the shopping trolley (a VW Polo) which now 6 years old. Nowhere near as good as the original Banner in the Morgan.