a replacement battery that may cost five times what even Halfords would dare to charge but does come with a slightly extended guarantee
The batteries are all the same.
I briefly worked for a parts distribution company[1]. When we had batteries to deliver, the order sheet would specify which guarantee the customer had ordered. We would then stick the guarantee sticker for that to the battery of the appropriate shape/size/capacity spec, and deliver it to the motor factor.
Vic.
[1] ADS, in case you were interested. They're gone now...