Had my ST in at the garage yesterday, took about 2 hours and cost £72.
This was taken from an ST forum, maybe apply to a normal fiesta.
As some of you may be aware, ive been suffing with this problem for ages now, making the car almost undrivable at times. Replaced the cables on advice from ford, which made no difference. After a bit of investation(pulling things apart!) i found out it was the Shift arm's nylon bushes that had gone dry/expanding, causing them to tighten on the pivot shaft. Anyway heres the guide, it might be a bit *****-ish, but it works!
1. Remove both plastic covers where the gear cables go tho the box. One will unclip off, the other requires 4x 8mm bolts to be unscrewed.
2. With both covers removed, press the button on the selector cable, and withdraw it from the ball joint on the selector shaft. Pull the shift cable off the shank on the shift arm(held on by a rubber bush, just pulls off).
3. Remove both cables from the bracket(the outer cable has a twist release mechanism, twist it and pull the cable from the bracket).
4. Now remove the spring E-clip holding the shift arm on it's pivot shaft, being carefull not to let it flirt off anywhere and loose it.
5. Carefully withdraw the shift arm off the pivot shaft, it will probably be extreamly tight(thus the problem). Try rocking it side to side and pulling it at the same time, eventually it will come off. The fact that it's that tight, is the root of the problem. Dont worry about the ball joint/bush of the shift arm that slots into the end of the selector shaft - it justs slides in and out(carefully stretch the boot of the ball joint over to allow removal).
6. With the shift arm removed, unscrew the pivot shaft from the gearbox, 22mm spanner fittment, be carefull not to damage the box.
7. The nylon bushes in the shift arm require reaming, or ever so slightly enlarging, i just used a drill with a drill bit the size as close as i could get to the diameter of the pivot shaft. Keep working the bushes until the pivot shaft slides nicely inside, being carefull not remove to much material from the bushes.
8. Once you are happy with the feel of the pivot shaft inside the shift arm(smooth), clean everything up and pack the shift arm bushes with plenty of grease/lube(i used copper slip). Screw the pivot shaft back into the box and tighten.