Okay, I'll give it a try again. I tried before and the little twerp zipped right on through! Perhaps I set something up wrong...
Thanks for your help!
I tried again as explained in the tutorial - make a sprite and paint-bucket it any colour but white. I tried whatever was first selected, which turned out to be black, as well as a blue, and my character parades right on through, with the Solid behaviour set.
What am I doing wrong? <img src="smileys/smiley5.gif" border="0" align="middle" />
So apparently resizing the sprite in the layout so that it is different from the original image is not the thing to do if you want solid to work...