2003 Boxster S extremely stiff clutch

[rimless]

Hello all, I have an issue with my 2003 BoxsterS, My clutch pedal is extremely stiff, I had a new clutch(genuine Porche ) installed by a Porsche specialist about 18 months ago and although once changed it was slightly better, I presumed that it was correct not having anything to compare it to. In the last few months the clutch has got even stiffer

I took my car back and he pulled out my clutch and took it to the official Porsche dealership, they checked and said the clutch was almost like new. He also stripped the entire clutch pedal housing to see if the spring was perhaps broken, however everything checked out fine. I have trawled most of the forums and most of them say if the clutch is very stiff it needs replacing, but clearly not in my case. I did read one forum where someone had the same issue and changed the master and slave cylinder I haven't requested my mechanic to do that as yet, I first wanted to check hear if anyone here had any ideas.

Thanks Rimless