The voltage seems to drop quickly to as low as 12.3v and on a couple of occasions the engine turns over but with not enough welly to start the vehicle.

A healthy battery is at least 12.6 volts after standing if the battery voltage drops to 12.3 volts quickly while the car is standing the battery is past its sell by date and needs replacing. Even if it has enough power to crank the engine it will have insufficeint power reserve to provide power where needed to start the engine.

The Yuasa one I bought early in June has stood for 3 1/2 weeks now and still reads 12.87 volts. The battery it replaced was 8 years old (Exide) and would drop to 12.6 volts after a week, it failed to start the car on just one occation and I replaced it.

So forget a starter pack and buy a new battery.

Edited by skidpan on 01/08/2017 at 09:58

Those hand held battery testers are notoriously unreliable. You really need to test a battery under load. You can do this yourself, all you need is a multimeter

https://youtu.be/AT_XmlwMVz4

If I were you I'd tell the shopkeeper that you want the battery tested properly with a load tester. I suspect he's using the hand held device as a way of deflecting warranty claims (which is odd, because he won't lose out anyway). If he won't, tell him you're going to get it done at a cost which you will claim back from him along with the cost of the new battery when it undoubtedly fails.

Before you go that route though, maybe email him to ask for it to be tested and expressing your belief it's failing in order to try and solicit a written reply which might be useful to you further down the line. Alternatively record your phone conversations - there are plenty of free apps. i use ACR.