Here is the alternator expert I used. Last time I used him was more than 25 years ago so he's got a little experience!
SandC Electrical in Harlow.
http://www.sandcelectrical.com
Yesterday evening Steve put it on a test bed and said nothing was wrong with it. After a conversation he said he'll strip it down in the morning and check all the components individually. This evening he explained all the components in detail and the situation of each of them. He'd also put it on 2 test beds under load to prove it.
If any of the components needed replacing he can find the alternatives through his extensive knowledge of Nippon Denso alternators and therefore avoid charging an arm and a leg.
I also showed my YouTube video of the noise to which he said its definetly not an alternator noise, perhaps I should of started here. He then said he'll share it with the mechanic that also works there. An hour later I got a call from him suggesting the secondary air pump.
Now that's what I call good service.
I did get charged a tenner for the strip down but it was worth it.