The clutch change on an ST170 takes a clutch specialist thats done many many ST170's nearly a complete day to preform a clutch change on an ST170, the gearbox has to come completely out the car and the DMF has to be changed, i would be very impressed if you managed it in 2 hours!!
Basically ford own the patten for the fly so you can only get this part from them, for a fly and clutch your looking at around £600 in parts, £250 in labour.
I spent £800 on having an aluminium lightweight fly fitted with a stage 3 clutch instead, i fitted the clutch myslef and it took me 2 days and its a job im familiar with.