Why not just replace it for the sake of £80 and not being stranded one day? You'll be good for the next 6+ years then.
The car has told you it's low so it's obviously not holding charge well enough to be deemed as reliable.
It'll be sods law that it'll start every day for months on days you don't 'really' need the car then on that ONE day when you really need to get to the airport or an important meeting, it'll be flat as a pancake
Agree on the sidelights though, something even my girlfriends 2006 Renault Clio has yet my Volvo doesn't....