Thanks John.
I'm having a local rebuilding ship look at it. Their initial comments were that the rotor is worn out and that they feel that Bosch, makers of the alternator, have designed the alternator badly and that more and more Boxsters will have this problem.
I'm going to try to go down to their shop and take a look at it now that they have it apart.
An interesting read for an alternator rebuild is here (the page below is on page 4 of 6 and has a picture of what a rotor is):
http://www.alternatorparts.com/cs130_sbpage4.htm
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