this is such a common problem and causes such bad running problems.

A new servo is about £120+ VAT, some sealant would do just as good a job. 2 nuts to undo the master cylinder and you can work it off the servo easily without disconnecting the pipes, and some sealant around the inside of the servo before refitting. You will probably notice a wafer thin o ring on the master cylinder anyway which is a piss poor attempt at making a good seal by Ford.

And above all.....you got nothing to lose by trying it!