JOE9090

Hi Guys, its my first post and my First Audi A4 1.8T 150 B5 2000
Basically i had an issue with the A4 day i bought the car on the way back.
I was driving on way back 50 mile trek and the car something beeping on computer but i could not read it as computer on dash is faded
anyway had very little to no turbo boost.
got home and read up the code on my code reader and it was reading code 17956 something to do with boost regulation valve. anyway i got told that i need to replace the n75 sensor (valve)
i did that today and i realized the turbo was still not engaging like it should.
i deleted the code and when i turn engine back on it comes back with the same code. and turbo boost is very little,
can someone point me in the right direction and also if theres anyone around Nottingham who can diagnose it. i know very little about cars.
Any help is appreciated and advice. i want to save this Audi as its really been looked after and a treat to drive.