It needs to be the right type and voltage Nick, and it looks like it will be a 24 volt lead acid battery type.
If so, you could look on ebay, but at the moment, the exchange rate makes supplies from the USA very cheap. If you go to the specialist supplier
Powerstream, then choose Battery Chargers from the list at the left, then choose Lead Acid Chargers, you can scroll down and see two suitable ones. The first very cheap one is heavy and will cost a lot in postage, so you might prefer the smart charger instead.
At the checkout stage, the postage cost is set at the highest rate service, so alter that, choosing either USPS standard or express rates which are reasonable and quite quick anyway.
Again, if it's definitely a 24 volt lead acid battery (SLA), you can also buy a suitable one from
Electrodrive in the UK, tel 07974 723996
sales@electro-drive.co.uk.
.