Thanks for your reply Peter.

If the earth connection was faulty would this cause the cars power to fail?

Also, would the voltmeter not read about 14.4 volts only if the alternator was faulty?

I ask this because both the alternator and battery are brand new so I thought that if a voltmeter was showing around 14.4 volts with the engine running, then I shouldn't be losing pwer as I drive around?

A 12 volt battery is not 12 volts, its actually 12.6 volts. But even a battery that holds 12.6 volts will not necessarily start a car if the plates are b*****ed or there is a calcium build up in the bottom, been there, done that. It can sometimes supply the volts but not the current required.

By putting a volt meter across the battery with a running engine all you are getting is the output voltage pf the alternator. This should be between 14.2 and 14.4 in a modern car although cars with stop/start and the special batteries can (and do) charge with a slightly higher voltage. Thus even with a totally duff battery you would still see the alternator voltage.

Same applies to the battery earth, the volt meter test proves nothing.

By all means check the voltages but clean up the battery terminals (and put a little vaseline on then before reassembling) and check the earth cable contacts to the engine and body.

Have you checked the the alternator belt is tight.