Turbo booster valve

We devopled a siren sound from turbo in which the mechanic said it was a dirty booster valve and after cleaning it the diag, said it was clear.Now the same noise is back again and its booked in for 13/11/13.Now he did say this valve is not possible to replace as Skoda only sell whole turbo's. Looking at UTUBE and hoping what I saw was a VAG turbo they showed a spring and ball bearing plus a cylinder in which the "valve "is pushed by the spring.. Now that showed the whole gizzo and to me it seemed easy to replace and prices were about £29 or so. Could some one tell me if this is so as when it goes in I will know what to say if they come up with a new turbo to be fitted. Many thanks.