Dead Battery

When we got back from holiday on the 1st of January the golf failed to start. Of course the moment the battery goes that flat the imobiliser makes it impossible even if the engine still turns (albeit very slowly). So I first tried to push-start it, as it seemed there was enough power left to power the ignition, but eventually resorted to jump starting. Initially I thought this was simply the result of the 10 days it had been standing and the drain of the imobiliser and the clock. But then last night it was the same thing again. I took out the old trusty battery acid tester and determined that 5 out of 6 cells have a SG of 1200 (just barely alive) and the 6th one is dead. After 6 years, I suppose it is time for a new battery. Interesting to see that some batteries actually still last that long. Then again, it is the original battery fitted by VW at the factory, which I think is sourced from Bosch, which iirc has a 5 year warranty.