Did the warming up of my existing battery technique and working fine - thanks very much for the explanation - at least I now understand why it was playing up!

i am using this near 20 yr old 90 for short journeys at the beginning and the end of the day so two cold starts a day and very little time to recharge the battery. It is usually discharged so much within 3 or 4 days so that it fails to start the engine. Glow plugs are fine so i'm thinking a more...