all of the below has been done by Isis Rescue in Oxford and by 2 mechanics, ALL tests were fine, Alternator is fine and the battery was giving our a normal charge
the only thing i
haven't done is Number 10
1) Try to charge the battery, ideally using a mains powered battery charger for a couple of hours, to ensure it is likely to be fully charged.
2) Allow the battery to settle for perhaps 15 minutes after charging so the terminal voltage reflecs its' true condition.
3) Measure the terminal voltage of the battery.
4) Apply some load (e.g. car headlights) and watch the terminal voltage for a minute or two.
5) Switch off all loads, start the engine, and, ideally, have someone watching the battery voltage while cranking the engine.
6) Measure the terminal voltage with engine at a normal idling speed.
7) Increase engine speed to a fast idle (2,000 RPM) and measure the voltage.

Add some electrical load (headlights, blower fan, heated rear window) and measure the voltage at idle speed.
9) Increase engine speed to a fast idle (2,000 RPM) and measure the voltage.
10) Check electrical connections to battery, starter and alternator and also all chassis ground connections in the engine bay for heat.