New battery fitted last night by Halfords and all my electrics are now working again. Bit of an effort though as he turned up with the Halfords battery, not the Yuasa I paid for but by this point I couldn't really give a s**t, so I let him fit it and will get the difference in price refunded. Same capacity, warranty and cranking power and likely still made by Yuasa anyway. He did have to remove the airbox to get it fitted as it's longer than the original, with a bracing block having to come out too. Makes me wonder if Ford had originally allowed for a larger capacity battery but then cheaped out at the last minute. Would explain why so many owners have battery issues...
Took it out for a proper spin round my local roads now that I have the confidence it's not going to crap out on me. I was somewhat worried when it started making an odd whining sound on acceleration until I realised Halfords man hadn't reinstalled the airbox properly, but that was a quick fix. Overall impressions are that the cornering ability of the car is night and day compared to my Fiesta. Instead of having to wait for the car to settle before you get on the power you can just mash the throttle mid-corner with plenty of lock on, and it just grips and goes. Probably a different kettle of fish in the cold and wet, but in the dry it's really allowing for some stupid cornering speeds.
Also figured out the cruise control now as it does have the ability to automatically raise and lower the speed based on road signs / GPS under the 'intelligent' setting. Not sure if I prefer it to the normal 'adaptive' that just follows traffic but it's still a novelty to have it automatically accelerate once you get to a NSL sign.