Thanks guys, there's a wealth of knowledge out there and I appreciate your assistance.
Let me clarify a little more the questions which was more about technology and original manufacture as opposed to good dealers. Today there are a number of alternative technologies available as opposed to the good old lead acid cell known as the "flooded" battery, such as AGM (Absorbent Glass Mat) and even LiOn. The AGM battery used a lot in Stop/Start applications, offers a low discharge in passive states ( sat in the drive all week) and can offer a longer (two to three times) service life as opposed to the normal flooded type. It should be a direct replacement for a flooded battery and needs no specially charging technology, just what's on the car.
Regarding our friends Halfords, they offer at £116 (includes trade discount) a slot in exact size replacement 80 A/H 800 amp cold crank AGM replacement for the S2 with a 5 year warentee (replace with new over the local counter). Halford batteries if I understand it correctly are badged Yuasa batteries.
The prices for the C31-1000 series of batteries on Tanya look competitive, but all the S2 series offerings are wet technology and 3-4 year max guarantee.
So any thoughts regarding AGM as opposed to flooded and the Halfords offering?