I don't know what batteries you have in the UK, but down under, you can buy a $150 battery or a $300 battery, the same size wet battery, yet one is 300 CCA, the other 800CCA, Then there are calcium one and gel. There are very ordinary brands and much better ones. I had very good experience with Delkor with a pair in my boat going for 12 years.
The longevity of batteries depends on charge. I have a medium size diesel generator for emergency, that gets used once a month if that, yet the battery is connected to a solar panel and is always fully charged.
Dual batteries present an extra challenge. If one battery starts failing, it drags the other down, very important to have a solenoid connecting both batteries that disconnects when it is turned off.
With excavators working the engine mostly at moderate revs and stop and start, charge the battery with an external charger fully at least once a week, if you want to avoid problems, or better still, have a trickle charger next to where you park the machine and plug it in every day.
With batteries, you get what you pay for, plus research the brand, some can be expensive duds.
Battery manufacturers, give you a hint with the extent of their warranty. 12 month warranty and you know you have a disposable one. 36 month starts to get into the reasonable quality.