Edit: Nevermind. Read the original post wrong. Get a new battery.
il get a mulimeter on to it, if the cars started the voltage should be about 12v+? if its started it should be running higher (eg. 14.4v) ??
Aye, get a multimeter across the battery just to double check. Diesels don't need an electrical source (battery) to run in their basic function once they have been started, so disconnecting it won't do owt.