As the title says, be careful buying a new battery. It might cost £150 or more. You don't know how long it's been sitting on the retailer's shelf. Leaving a lead-acid starter battery unsued for a fair time can damage it by crystal sulphation. And you won't know until a year or more later. Nowadays batteries can't be supplied 'dry' of acid as they used to be; the retailer filled it with acid at 2.8 gravity. All batteries are sealed now.
So what can you do? Take a power test-lamp with you (mine is an old car headlight of 55watt with some leads soldered on). The battery should light the lamp for a minute or more. Or you could be sophisticated and take along your digital voltmeter and measure its voltage. If it's less than 12.6volt, refuse it and ask for another.
I can report that a silver calcium-calcium battery I acquired recently is astonishing as regards lack of self-discharge. It is still on its surface charge after a week at 12.9volt. It should be 12.7 by now. It's made by Ducelier.

Leedsman.