With Paul Lupus's help I've 'solved' this problem: it is a designed behaviour of the Arachnophilia program and in no way relates to java virtual machines, faulty installations, packages or any of the other angles I tried. Now to undo all the things I've done messing around with my java installs!- lastly, you could ask Paul Lutus about which java methods are used for the undo function you are missing, and try to get nearer to the lines of code in either arachnophilia or the java implementation you use to help track down the problem. Maybe arachnophilia has a unit test suite, and you can try if these tests run fine. As arachnophilia is not free and seems not to come with sources, your possibilities to check the problem seem very limited to me.
Undo and redo on Arachnophilia will only work on cut, paste and copy. It turns out that my undo/redo testing of a new version of Arachnophilia (5.3) did not adequately cover these scenarios. I was an active user of Arachnophilia 5.2 but couldn't narrow down why undo/redo was sometimes letting me down... I had assumed the functioning of undo/redo was standard between text editors. Then when I tried 5.3 my test case was a blank document type type delete undo etc.
I am okay with Arachnophilia not undoing key strokes but the delete key stroke is a special case. It can take out whole chunks of text or even an entire document. This needs to be undoable.
So I will go back to gedit for the time being. The combination of gedit 2.18, sitecopy and web-developer extensions for firefox are a pretty good way of working for me.
As far as providing cross-platform html training I will have to look into alternatives. Java is the obvious environment for this sort of work. GTK/GIMP is another great angle. I will check out jedit as suggested
...but those html macros and wizards in Arachnophilia are really handy reference points for beginners... Maybe I will get lucky with Arachnophilia 5.4...
Thanks to Henning for the help. Graham E