This morning on the motorway, I floored it in 5th gear for a few seconds, and the clutch started to slip. It wouldn't do this in any other gear and is perfectly OK this evening. It could be that the clutch is at the end of its life (54,000 miles) but I think that something else is wrong. The clutch pedal is now much lighter, smoother and no longer juddering - I'm wondering if a spring has gone or something. I would appreciate any comments.

P.S. How much is a replacement STi V3 clutch, fitted?