Having gone through a nightmare saga with mine.....my money is that it is a slave/ hydrolic issue you've got, rather than mechanical....I ended up having to go through a process of elimination, which for me, ended up being 'worse case scenario' with the internal (there are two, external and internal) slave needing replacement, but hopefully you'll get away with the external slave simply needing bled and refilled with new fluid. If this doesn't work, then they will replace for a new external slave, which in most cases should work.....if your as unlucky as me, and it is still doing it after the first two steps, then sadly it indicates that the internal slave is fooked and needs replacing, which is a mahoosively big and expensive job as engine needs dropping, gearbox, clutch out. Because this job is so extreme, it is recommended (by other owners/ forums etc) that you go for water pump and new clutch at the same time as it is all taken apart and best to do all at once rather than risk having to do again! I went for the whole shebang and it cost me over 1k. Schummy was like a new car after it all, but it was about 6 months of hassle, grief and serious money to get there.
As has been said the fact that it is good to start with, and then gets worse throughout a journey, is defo an indication that there is a leak through seals which recovers when standing still again. My issue started off exactly like this but my issue was getting first, and over several months just got worse until I was eventually stranded with no clutch to get anything....to a garage it went!
My advice would be to initially flush, refill and bleed the system with new fluid, either yourself or garage. If this fails, replace the external slave before going onto the more difficult expensive stuff....