Thanks for all the information guys. It's all gone in and is very helpful.
I have the head all stripped down now and am about to go off to work with a large bin liner of bits ready to be bead blasted. As you can see from the photo's it did need it, it was filthy!
The second photo shows the damage to the fins which would quite badly reduce the airflow.
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My Brain urt's!
Continuously in Awe! of (H)Al, the Photonlexicon Font of Complete Knowledge - The (H)Al'PL Database of complete puss that no one needs to know or ever trusts as he ain't really got a Scooby doo about now't!