I just found the AA guy's readings from when he told me that I needed a new battery.
This was having run the engine for about 10mins from starting after it was flat:
measured 380 CCA
rating 650 CCA
So what do these mean?
12.36V is alright...mine sags to about the same value.
Measured CCA is the 'cold cranking amps' which the battery was capable of providing, basically the peak current it can supply across almost a dead short. Your battery is rated for 650 Amps for cranking but it only managed to provide 330 Amps.
After seeing those figures I agree with the AA
Although the battery is relatively young, these sorts of figures are typical for a battery which has sulphate crystals on the plates, preventing maximum current from being generated. You don't need to lose very much capacity for the battery to struggle to start an engine.
The worst thing for any lead acid battery is for it to stand with little or no charge in it. Even sitting for a week at 11V will substantially reduce it's ability to give high CCA in the future. The lower the voltage the faster the rate of growth of sulphate crystals, which is why batteries kept on a maintenance charge, or a trickle charge always perform better and last longer
There are some chemicals yoiu can buy to rejuvenate batteries, I've never used them myself but I know a couple of people who said it helped them out. Some people use 'de-sulphators' , which use high frequency pulses to fracture and loosen the crystals.
Maybe though it is time to get a new battery and retire this one either as a wind turbine or solar panel backup supply, or just put it in for recycling