Turbo-brown

Hi there,
I've been lurking for a while,trying to find possible causes for a problem we're having with our car.
It just doesn't want to rev!
If you're doing 50 and you bung it in 5th gear and floor the throttle, you're met with pathetic acceleration and a mild juddering which you don't really feel so much as see when the sun visors start wobbling.
Any suggestions or pointers gratefully received!

P.S. sorry it's not a Focus,but it's made from the important bits of a Focus ST!