I doubt the earth strap would be doing it to be honest. Mine was completely off for a fortnight on my Puma and it didn't make a blind bit of difference.

There has to be a reason for your battery draining (if it is the battery) So it's either at fault of you have a drain somewhere - dodgy aftermarket alarm, light etc.

I've had mates that have bought a battery that just won't hold a charge. The tests all show it to be fine, but when swapped for another battery all was well. Just because your "new" battery is only 9 months old, don't discount that being the culprit. Get it swapped under guarantee.

Good luck.