Many thanks Nige005, just to clarify......
The noise only stops because the revs lower as you change gear. If you hold them at say 40000 and hold the pedal down whilst stationary the noise is present.
Is this a DIY job or one for the experts?
Can anything be added to the Gear box oil to help?
Will I do any damge to anythng else other than the release bearing by leaving it to get worse?
Hard to diagnose Jimny, try the other suggestions first but have to say that spraying the belts with WD 40 doesn't seem like a particularly good idea, it can only mask what is wrong but not cure the underlying cause such as a squealing Tensioner etc. If it was something to do with the belts etc I can't see why operating the Clutch would make any difference to the noise as the engine rpm's would still be the same.
If it is something to do with the belts and the Clutch doesn't really make much difference it could be things like the Water Pump, Alternator, Steering Pump or as an outside guess the Oil Pump. Or as suggested it could be an incorrectly tensioned Belt.
If the noise is from the gearbox you could replace the oil with some Wynns stuff, there is a recent Thread about this and if I can find it I'll post a link on here.
If it is the Thrust Bearing you run the risk of complete failure which will leave you with no clutch, not good. I think that the risk of other damage is probably not too great a worry 'cos if you replace this you would replace the whole clutch at the same time. Replacing the clutch isn't a job for the faint hearted, my view is that it would be best left in the hands of a good indy.
Does the Clutch pedal feel stiff or jerky ? Sometimes this can be a symptom of a failing bearing but it could also be other things so not really a foolproof way of deciding what is wrong.
On this point I would be dropping in to your local Indy just to get his idea of what it could be, very difficult to diagnose here on the Forum.
Hope you can find it and that it's not a wallet busting problem.
Just noticed that you are in South Yorkshire ( I'm a Barnsley lad myself ) and if you don't have a good local Indy I can recommend Tom and Phil at BLS in Lincoln. Not too far from you and I'm sure that if you dropped in they would be happy to give you their thoughts.