Is there any fluid loss from the reservoir? The Defender uses a very strong pedal return spring so you may not notice too much difference if the hydraulics have failed.
Any nasty noises? The release bearing might have failed, but it would usually make some nasty graunching to get to the total failure stage!
Have you been wading lately? With no plugs? I had the friction plate stick on the splines after getting "wet"! Eventually, it freed up on it's own. Well, after I replaced both the slave and master cylinders
