Thanks for your input Brad-B5 it seems like a common fault that a lot of Audi's and other turbocharged cars suffer from the variable vanes gumming up with carbon and flagging up the fault in the ECU resulting in the car going into Limp Home.
Sometimes, flushing through with proprietary cleaners helps but if gone too far then replacement turbo or cassette is required.
Other areas to consider are also as already mentioned, boost leaks (intercooler, hoses) and also boost solenoids going faulty.