FWIW I actually managed to get an expected result from doing a PixelMath operation!
I was confused by the special PixInsight "style". I've been trying to put it in words and can't, but I've read of it. "Is this part of the process or is it an example", for instance. Or, the way that ctrl-A does not do anything to anything, just looks like it does. Remember I had no experience with PixelMath until just after the new version in 1.8 came out.
Anyways I'm very happy now and can continue from here. This forum and its people are great :-)
James