Hi mate...
i bet it only had a years warranty too.. typical of todays world.... but it could be a few things..
duff battery or bad contact.... when it starts ok, go & get the battery tested again.. they should tell you if the voltage is abnormal or not.. and if the charging is still ok
i see its charging ok - then its not the power steering reservoir leaking onto your alternator & shorting it out (common fault with the MK1 & 2 LS!)
or it could be a well worn wiring loom causing something to stay on (like the old phone in the centre), or the boot light. & make sure the boot light switch is working correctly... you'll have to open your boot & then put your keys in to ign.. go to boot & find the switch & press it to see if the light goes out.. or take the bulb out for a week & see if the battery still runs flat..
The wiring loom well known for it is the boot loom, it runs thro one the hinges...but it has been known to happen in the front doors too... you'll have to gently expose the wires to see...
Good Luck... If not, you could always buy my LS :)