Our car is stop-start, it hasn't performed this 'feature' for many
months. I believe this to be a symptom of the battery not being at it's
best.

It was clicking the other day rather than cranking the engine over,
worked OK upon retrying. Taken it for a couple of decent runs since
(because it's been largely short runs all year, due to the obvious),
it's been OK.

I happened to have a meter in my hand today so checked it and it's
reading 12.1v which is quite low. Battery life generally being 4-6 years
and us having had the car 4.5 years i'm thinking shall I just replace it?

Advice is to trickle charge it first but I haven't a charger and I don't
want to buy a charger *and* a battery.