Had one do that on a Shogun a couple of years ago, two year old reputable make battery checked out fine on an expensive battery tester, cold cranking amps were fine but would lose its charge over a week if not used. New battery fitted and all sorted.
An old Land Rover has very simple electrics, the main culprits to look for are any aftermarket gadgets or faults within the alternator. Easy way to find the issue is to disconnect a battery lead and connect a multimeter set at 10A in its place, then find what is causing a draw by pulling fuses and the alternator plug until the meter reads zero. Do not switch any circuits on while the meter is connected for obvious reasons.