Started to develop a high pitch screech on accelerating up to certain revs then stops, loads of power, no faults showing, car drives ok,

From what I've read on the forums (it hasn't happened to me yet), that sounds like a boost leak.

Check your top boost hoses - TDV6 or TDV8?

And also check if the hoses are still on attached to the Turbo.

Also could be cracked rocker cover manifolds that are letting boost pressure out of them.

The pipes if a V8 tend to go soft and rub through and leak boost.

Have a good look around the engine bay for signs or any blackened oily mist thats on any pipes or plastic manifolds x2.

fingers crossed its a cheap fix for you.

Dean