Finally got round to fitting a new belt. The smaller belt running the aircon compressor didnt seem to be the issue, it was the larger belt.
Changed the larger belt which cleared my issue when we started and tested it. I drove home and to my grief the noise is back!
Could this possibly be the tensioner at the top that you have to slacken to get the belt off? If so how can i test it as looking around they are about £50+ and dont fancy changing it if its not that.
Any other ideas as to what it could be?