ill go with clutch fork failure
i'd be surprised if the clutch fork could fail, it's not as puny as the ones fitted to defenders and discos
plus if it did fail i'd expect the pedal to go solid as the slave reached the limit of it's travel just as it does if the release bearing collapses/seizes and melts the carrier
i wouldn't expect there to be a pin prick in a (the one and only) hose, maybe a leak in a joint which would show as the fluid in the reservoir vanished and leaves evidence around joints or on the bulkhead, possibility of master cylinder failure (leakage) but that generally leaves evidence down the side of the pedal and in the footwell, the slave cylinder "could" have failed so it would be harder to see evidence of leaking as it would then leak into the flywheel housing thus you would first be looking at the wading plug hole to "hopefully" see traces running out of there (remember any rear crank leaks also drain out of that hole) that is "if" the sherpa engine has a wading/drain hole in the flywheel housing
to me it sounds like they keep putting the work experience guy on this job, i can change clutches on the S3/2a/2/90 on my own in about 6 hours outside in the open and that's with all the floor plate/tunnel screws/bolts rusted solid and needing grinding off carefully along with several cups of tea and sarnies to stave off the hunger/boredom
there's nothing simpler to set up than a S3/90/disco clutch, the S2/2a is slightly more complicated due to the external linkage/cross-shaft from the slave cylinder but also has minimal difficulty
personally i'd guess that the owner of this vehicle is going to be spending an awful lot of money at this garage over time, money that in my opinion would be more wisely invested in a few tools and a couple of manuals as there's little that is particularly difficult about maintaining/repairing Series landrovers