Battery

After putting up with apparent slow cranking, I think my battery finally gave up. Last night at the pub I had to get a jump start (lucklily was carrying leads!) And this morning it really didn't want to start but did after some slow cranking (great engine!) Not expecting it to start at lunch time!! - but am going to go get a battery (I know its the batter and not connections as when I applied the jump leads cranking was excellent and it started straight away - thus negating any issue down the wire somewhere) - the batter also has 'charge by Jul2003' on a sticker on top, like a best before date so I am assuming this battery has to be 6/7/8 years old.
Anyway - knowing nothing about battery's and Amp Hours and cranking, what am I looking for battery wise. Sorry for the simple question but these are the things I need help with! (Tried a TSN Battery search and couldn't find a list)