So, just for some closure on this thread: it looks like the old battery was the issue, I've been monitoring the battery charge of the new one, and although it drops to ~12.48v overnight, a bit of use has it showing 12.7v afterwards.
Slightly concerned that the max value I've seen with the engine running is 13.5v and it regularly drops to 13.2v, but I think this seems within range as these cars apparently have "smart" alternators.
Also, my intermittent parking sensors are now working every time which is another telltale sign of a dodgy battery wreaking havoc with the electrics.
PZero's fitted too, and back on the road - these cars are an absolute joy in this weather
Slightly concerned that the max value I've seen with the engine running is 13.5v and it regularly drops to 13.2v, but I think this seems within range as these cars apparently have "smart" alternators.
Also, my intermittent parking sensors are now working every time which is another telltale sign of a dodgy battery wreaking havoc with the electrics.
PZero's fitted too, and back on the road - these cars are an absolute joy in this weather