Could just be the variable vanes in the turbo are not moving as they should due to carbon build up. (This can happen if the full range of engine revs is not used, for instance if the car is driven as one would drive a petrol car for economy, gentle acceleration and using high gears/low throttle openings.) Several reconditioners do exchange units, which will be cheaper than new and IF properly reconditioned should perform just as well.

Could be a split or damaged inlet hose - not impossible. Having said that, most AA people know common cars well and if they think the turbo has gone it probably has.

Only answer is to get a proper diagnosis. Going rate for a recon turbo is £400-£500+VAT+fitting. A new one for most of the 1.9 TDI range is about £900+VAT.

Most passenger cars only really spool their turbos for very short periods of acceleration. Industrial/heavy duty diesels run their turbos at 20 psi all day long and they don't have many problems with them.

All Cummins diesels are turbocharged, I believe they guarantee even their medium duty diesels ( B and C series) for 250,000 miles. (They use Holset turbochargers, which are engineered to last at least as long, assuming oil quality/cleanliness and air filtration recommendations are maintained.)