I'm still very interested to hear from anyone with an XR5.
Comparison with other peoples cars is one way of convincing the dealer that a problem exist with my car.
Also, I spoke to a local brakes specialist this morning.
He suggested the problem was an internal seal somewhere allowing fluid bypass, but without fluid loss from the system.
To solve the problem, he suggested start with the master cylinder, then check
ABS etc.
Failing a satisfactory dealer response, next step would be Fords Customer Service Centre.