You could try bleeding the clutch slave cylinder see if that helps, there's a guide on here somewhere iirc, but if that doesn't work I'd suspect somethings on it's way out and will likely mean a gearbox off job to fix! - So basically a new clutch job.
Also check all the vac pipes going to the master cylinder are secure, they can cause some strange issues if they work loose or have a crack in as I found out on one of my previous cars!
Clutches/clutch components can let go at any time/mileage really. A low mileage car, unless it's only been driven at weekends, will likely have seen more clutch use (from short journeys/town driving) than say a car with high miles (more motorway work). Not always the case, but just don't always assume low mileage = plenty of life left in the clutch, you never really know how the car's been driven unless you've owned it from new?