1991 Golf Clipper not starting

I have recently bought a 1991 VW Golf Clipper 1.8 Automatic. It has a manual choke and a new battery fitted this week. The problem I have is the car won't start on it's own but is fine when jump started. I thought this was a problem with the battery so bought a new one. It worked first time. Now after a few days I have the same problem again. I assume something is draining the battery. I have had a new stereo and Immobiliser fitted, however I had this problem before that. There is an old immobiliser and alarm on the car that was already there. could this be electrical? something to do with the alternator?

Suggestions please

M