So I have been following this thread with interest. I was wondering how long Thumper's battery would keep going (original fit 2012) and needless to say, with my son driving the car yesterday, we saw it's demise.
Following the art of good British compromise, I have opted for one of these:
https://www.tayna.co.uk/WCP30-Westco-Platinum-Motorcycle-Battery-YIX30L-BS-SVR30-P7811.html
400A CCA as compared with 300A for the old Banner battery. It will still work with my Ctek battery conditioner and may only need a modest replacement of the battery studs, as it is a bit taller.
I'll post how it goes.
Tim