Sinking Clutch Pedal

SWMBO has just informed me that the clutch pedal on my 2002 Fabia 1.4 16V was down on the floor this morning, but once it was pulled back up the clutch was fine. I have done a search, but nothing much has come up although there were references to a dodgy master cylinder. Anyone got any idea of cost for a new master cylinder and how difficult / easy it is to change.

Thanks.