Hi Steve
Is the slave cylinder piston moving (acting on the clutch release fork) when the pedal is depressed?
If it is, then there must be fluid in the system - air alone will just compress and not cause any movement.
But if not (and there is a fluid supply to the master cylinder), then I would guiess that the master cylinder is gone. It isn't difficult to take apart and take a look at the seals on the piston and the cylinder itself. Note that the fluid in the reservoir on the brake cylinder is for both the clutch and the brakes. There's a clear plastic hose taking the fluid from the reservoir to the clutch master cylinder which you can seal off before removing the master.
There aren't any cheap alternative parts for the master and slave cylinders, unfortunately. Check the usual suppliers
But unless the piston or cylinder is worn, you can replace the seals on the piston in the master cylinder - as described in this thread (incidentially with almost the same subject title):
http://www.matrasport.dk/forum/index.php?topic=661.0- Anders