Big Thank you to everyone who replied.
We found the issue in the end.
What had happened there was an RF rogue client who had statically configured the IP address of the NPS box!!!
All radius requests were going to this client.
As a result the lack of response meant the controller was put into a "dead server" group.
When we tracked down the client all was well.
Chap called Manish from Aruba TAC was excellent in helping out.
CHEERS
Pete