Zeus,
I would think its is your alternator which is causing the problem,that would drain your battery if there is a fault in its internals,it would also cause bulbs and other items to fail if it is producing to much electrical power when the engine is running ( glow plugs,starters etc).
I have had strange experiences with faulty alternators in the past and they cause all sorts of perculiar things to happen.
I was recomended a good Dublin based auto electrician recently,cant find his number,but will mail you when i can find it!!!!!!!!
Springer