got to the point now where the kids playing football on the street move onto the path even before i turn the corner because they can hear me squeaking up the road!!!

tried talc and that halfords sticky belt stuff but no change.
a mechanic has had a drive in it and says its not an air leak and doesnt sound like a belt/pulley/tensioner sound but he doesnt know what it is! really stumped now-no idea where in the engine bay it comes from as it only dose it when its motoring

Noises as you say are very difficut to pin point particularilly so from here, however here is a bit on turbo as requested.

Turbo bearings generally have movement in them, and quite a bit, when the bearings go however the play is such that the blades touch the inner casing slightly and feather them, once they are feathered they lose their balance and make a noise but usually whisteling, because of the uneven shape of the damaged blade. Once the turbo gives completely it chucks the fragments of blades into the engine and it's not what you want.

This normally ocurrs on boost pressure just as you describe, and would disapear on lifting the throttle. The noise you describe however does not sound like a bearing/blade issue but if you take the hoses off and look in carefully you can examine them for the damage mentioned.

Sometimes the bearing gives up and ****es all the oil into the engine and causes a fire, without a noise, but that's an obvious fault, like a rear smoke screen.

cheers, ill have a look at it tomorow