[Symes]

+1 for checking the earth strap (low front of engine under starter). My battery would last a week or so after an off car recharge, its a cheap Tanya one and 4+ Years old so I was looking for replacements but thought I'd check the charging circuit first.
The alternator was outputting 13.5V checked with earth on the engine, but 12.5V when earthed to the body. The earth strap to engine surfaces were slightly dirty looking but a few minutes with some emery cloth got the surfaces nice and shiny, smear of copperslip on reassembly. Output measured with earth to body returned to 13.5V (rising with revs as expected), starter turns over quickly again and the battery seems as strong as ever!