I have a Ford Focus 1.8 diesel 2003.

The air con is intermittent when switched on.

It will work immediately on start up and operate fine for 10 - 15 minutes then blow ambient air. If I switch the air con off when driving and back on after 20 minutes it works but only some times.
On one day it worked for the duration of my journey the following day it did not work at all only blowing ambient air.
I travel 55 miles each way to work the majority of the journey is motor way. I would appreciate your assistance.

 

Could be low pressure in the system, as they have a pressure switch built into the system, so if there is a leak then it will cut the air con off to prevent damaging the pump if it is run with no lubricant in it.

Only way to tell though is to have an air con specialist with the right tools look at it as its not something you can do at home really!