Disco 2 Battery

My 2003 td5 battery drains for no apparent reason. If I use the car every day it seems fine but if I leave it for any length of time it's dies any clues?

Sounds like its getting on a bit.
Have you had it checked?

Disconnect it and fully charge it. Leave it for a few days a and measure voltage. The result will be a new battery or a lengthy investigation by pulling fuses.

is the charging system OK, measure voltage with engine running, it must be around 14V, fubar'd alternator beside the fact that is not charging the battery well has a parasitic drain too if you have a multimeter measure the drain but to be relevant leave the bonnet open and measure after more than 1.5 hour after the ignition was off without opening a door to wake up the SLABS, see this

I have a cigarette adapter and it show 14v when engine is running and 12v when it's off. Everything works fine while the engine is on. Does that mean the alternator is fine? And the battery bad??

it means only that the alternator is working... period, you have to measure the drain for further conclusions