[Zebster]

I doubt it's the auxiliary battery that needs replacing. Probably the main battery is perfectly serviceable too, but its loss of charge capacity over the years is making your battery monitoring system think that there's a problem.

I put up with 'low battery' warnings and the loss of stop/start function for a couple of years before finally replacing the main battery last month. It all worked fine if it got an overnight charge but soon reverted to warning messages and no stop/start. All is good now.