Winter I almost here, according to TDSA, so any weakness in the battery will soon show, may be of interest to some.

http://www.ebay.co.uk/itm/Land-Rover-Batte...500wt_1057

Pay attention to the manufacture date... It is stamped, in relief, on the battery.
An unused/new battery, but old in the same time, is not necessary a good one...
In general, the batteries need to be kept/stored charged and periodically recharged and, if it's an old one, are you sure it was properly kept/stored?
See the thread about the car batteries and DiscoGeorge's comments...