What a fool!

In the cold light of day, the problem became apparent. My suspicions
were alerted when I noticed a wheel nut missing from the nearside
front wheel. (My wife had thought it was coming from her side!)

Yes, the wheel was loose. I consider myself very lucky it had not
decided to come off completely. I am also very lucky that this has
not cost me anything to put right! (Well, I'll need to replace the
lost nut. It was a locking one as well!)

Keep checking those nuts. I certainly will from now on.