Turbo booster valve

Today our Octavia Elegance 55 reg 53000 miles went in for its 4 year major service ie cam belt and pump, plus a siren sound from the turbo which the mechanic last week said he had cleared the carbon off , it only lasted 3 miles before coming back again . Skoda rang me today saying you cannot replace the booster valve as its all part of the turbo. Price quoted at moment is £2000 for turbo. Going to the garage I tried to give it some welly but it would only get to around 60 mph.We do not do many miiles as can be seen but this turbo going at 53000 miles seems too much.Can any one give advice on this. They are keeping it for the whole week so far. As its a main dealer they do not seem to want to strip it down and replace any parts.Thanks.

Edited 11 November, 201312 yr by pastiche