Clutch pedal drops when driving down bypass but the car is fine to drive around town 

There doesn't seem to be any logic in this, to me, so I'm asking for more information for others to comment on?
When you say drops on the by pass is that with or without any pressure from your foot?
How is gear changing affected, any difficulty and which gears?
What is the car's  history, any work done recently?
What mileage, could the clutch plate be worn out?
Could there be air in the system  - does it feel spongy or some other strange feeling that is different between bypass (at speed) in town (slow speed) and different from what it was before all of this started?

The bypass is likely to mean a ring road.  So constant speed cruising without any clutch use.

Suggests that that seals in the master or slave cylinders are weeping, and only able to maintain pressure when they're being regularly pushed and returned during town traffic driving.

At best it needs a master cylinder, at worst it needs a slave cylinder, and by the time you've got the gearbox off, might as well change clutch plates as well.  On a 2003 Fiesta, the cost of a clutch kit fitted at a garage is likely to be more than the value of the car.