Thanks guys and sorry to take a week (almost) to reply again - I had a bit of a minor family crisis and had to go away for a few days.

I think my problem stemmed from switching between layout managers several times early on. Having wasted several hours on the size problem and never actually succeeding in changing the size programatically at all, I decided to start the GUI again from scratch this morning. 2 hours later and I am finished and all is excellent. I must confess to finding building non-trivial Java GUIs very hard work, even using Free Design with Netbeans. But them I'm hardly the first person to say this.

Thanks again.