Battery draining somewhere/somehow

[japiedog]

I have had a starting problem with my '59 plate Trend for some time now.
Originally, once she had started in the morning , things would be ok for the rest of the day.
I had all glow plugs replaced and fitted a new battery . The alternator was checked when the battery was replaced.
She was only hard to start if left unused for a few days.

However, now the situation is reversed. She will start easily in the morning, but, after even a short run the battery will have discharged and she'll be
impossible to start without a jump. No alternator warning light. If I leave her for an hour there will usually be enough juice to start the engine (if I'm lucky ).

Has anyone had a similar problem and/or can offer a solution ?

[Kenton]

I would have the battery drop tested and the alternator again. Then maybe conduct a voltage drain check to make sure?

[Altransit]

I would be having a look at the starter motor, it may be partially seized when hot