Update !
I've took the air con clutch off to measure the offending bearing, it was noisy and felt slightly rough when spun round.
Anyway if anyone wants to save a few quid and do a bearing swap rather than changing the whole ac pump, here are the details.
Its a 30mm x 52mm x 22mm bearing available from Henderson Bearings Item description
Nippondenso 30BG5222-2DSE TV12, TV14 PFI/KYK Air conditioning Bearing .
Easy job !
1. Take off the plastic fan cover for access.
2. just push back your belt tensioner to take the belt off.
3. Remove the small bolt from the front of the ac clutch.
4. Pull the clutch off.
5. With some circlip pliers remove the circlip from the front of the pulley wheel.
6. Pull the pulley wheel off.
7. Now you can see the bearing easily on the reverse of the pully wheel you will notice it has some machined metal keeping the bearing from coming out. Just knock these bits of metal away and remove the bearing.
8. Put new bearing in and using a punch or similar tool to push some of the metal over the corners of the bearing to stop it from moving.
9. Put it all back back together again
Thanks to all who responded to my post and I hope this will be of some help to others.
I'm aware this is a common problem for a lot of vehicles, not just disco's !