As a first step I wold take it to your nearest Halfords and ask them to do a battery and alternator test.
It's free, using what seems to be a pretty foolproof computerised tool so it's independent of the skill of the actual operator....
An intermittent fault may be more tricky but you will still get useful info. Before going that I would clean and tighten all earth and battery connections as advised btw.
Cheers
Joe
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