I had the same clutch symptoms and cured it by changing the master cylinder on the bulkhead.
Mine cost £90 but only because I wanted it quickly and popped down to euro car parts. Same on eBay would have been £69 ish.
Simple to change too - its even possible without any tools as it just clips into place and comes pre-filled with fluid so doesn't need bleeding. Mine was a 30 minute job, 20 of which were spent splitting the pipe union thing.
Definitely worth a shot before hauling the gearbox out...