The ST170 clutch fails prematurely, some after just 20k its just a matter of time. The reason for this is one of 2 things
1. Ford fitted in the factory a smaller friction plate to the earlier models, later changing it to a larger one in 2003, this was a recall in the USA but we never got the recall.
2. The dual mass fly wheel, the bushes on the flywheel wear then causes movment which causes the clutch to slip, or you get a horrible chatter sound like your slave bearing is foobar but its actually the two plates of the fly hitting eachother.
When i removed my chattering flywheel it was in a right state im suprised it stayed in one piece. You are in a very small minority if you have not changed your clutch by 60K.
It WILL fail and the replacment parts are VERY expensive