not holding a charge

Ive got a 1986 2.5 diesel 90 when i go to tart it in cold mornings it turns like its using a flat battery but its a new one the starter has just been replaced to when i get it going after charging the battery it runs fine nothing is left on and in the morning its dead again its got a new 644 heavy dutie bat any one got any ideas (or a gallon of petrol and a claim form)

Alternator not working? Slack/worn belt? Loose connections on alternator or battery? Short circuit on starter motor live? Tracking across battery terminals?

Few things to look at first

with the keys in yer pocket disconnect the battery pos lead and put an ammeter set on the 10 amp range in series between it and the battery terminal - anything more than a few hundred mA isnt good - check that any additional electrical ietms that you may have added are wired into the right side of the ignition - dont dismiss the posibility that the new batter may be at fault