Jim and MD,
I have tried to adjust the offending properties (size, font, font size
etc) in the properties tab BUT ALAS this problem is something really
odd - the 'corruption' persists.
Open to any other suggestions ...

(Monday 11 Aug) New info - the PC that was doing this to these
controls isn't doing it today, so it's intermittent. It'll be nice to
know why it happens though.