but the mileage is still very low so I put it down to that

Hmm... I find this is a very rare thing, but my Ibiza has seen 9,900 miles and the Polo around 55,000. The slightly unnerving one was the Fabia that did it when I was taking it out on a test-drive.
 
next time it does it just lift the clutch up a inch and have another go rather than full clutch up and spinning the box before retrying


Mine does it maybe 3 or 4 times a year so no big deal at all