Typical eh Chris.
A quick check you can do to eliminate ancillaries is remove the auxiliary belt by using the tensioner, then start the car. The alternator, water pump, idler pulleys, etc won't be driven by the belt. If the noise is gone it's one of them. You can then spin them by hand with the engine off and see which one it is.
If noise doesn't go it's nothing driven by the belt.
DO NOT run engine for too long with belt off as you have no water pump for cooling.
PS Not sure how involved removing the belt is on a Turbo. On a Carrera it's simply a case of removing the airbox then using a spanner on the tensioner, and slipping the belt off. My GT3 was equally simple so as long as none of the extra Turbo gear is in the way, it should be easy.
A fellow turbo owner may have some other ideas. Good luck.