Is there something inherent in Morgans that drains batteries ?
If fitted then immobilisers and alarms will drain the battery. Many modern cars will only last a few weeks before being unable to start.
If there's no outside power source then a battery disconnect switch is the solution.
I use a Battery Brain on my M3W so I can disconnect/reconnect without having to take the front cowl off.https://www.batterybrain.co.uk/