Clutch slip on a classic Impreza Turbo

[HonestIago]

This evening coming home I put my foot hard down in 4th doing around 60mph and the revs just wound up to almost the redline with no actual acceleration. I backed off and it was then fine the rest of the way home. Does this mean my clutch is on the way out? Is it likely to just fail and leave me stranded at some point soon? Or will the slipping just get progressively worse? My car is running about 300bhp and has 97k on the clock. Input appreciated!

[HonestIago]

The car is going in for it's MOT and a service in a fortnight so I suppose I'll just have to drive it very gently in the meantime and add a clutch to the list...*gulp*