Hi, Had a similar problem on 2002 disco V8, two things to check, 1) if you have towbar and 2 sockets then check power connection to aux plug, mine was situated above top right hand light cluster and was giving a power drain. 2) there was a 300ma power drain through slabs unit (common problem I believe on early slabs units). One way to rectify this is on one of the forums and involved working on the slabs unit in the footwell, as it is a bugger to get to I looked at the other end and worked in the fusebox by the battery. Current drain is via ABS fuse (fuse 11 I think) So basically have taken the ABS fuse out, taken the feed to the fuse and fed to a std LR relay in terminal via an inline fuse of same current rating, taken relay out back to other side of fuse. Power to relay taken from ign switch so when starting up there is power to slabs unit, when turned off relay cuts circuit and prevents current drain. If you have air susp as I do then I let the disco stabilise for a short time before switching off.
Hope this helps.
Regards
Baz