My Felicia revs before picking up speed in higher gears, particularly 5th but also to some extent 4th and 3rd. I'm guessing that the clutch is slipping. Does this mean that I need a new clutch?

In a word YES

Is this a new trend? This is about the 10th post I've read today where the OP has solved his problem before posting!

A little affirmation can be nice, especially before shelling out a couple hundred £ for a new clutch