HarryT: thanks for suggesting decompilation, I had not realized this was possible, or that such useful tools existed.
DiapDealer: your underline sample mobi works on my Touch, but when I changed the "filepos=XXXXXXXXXX" to "href=......" the underlining came back. I don't understand how your method is meant to work, is it the enclosing <p height="1em" width="0em"> tag?
After decompiling the "Bad Pharma" sample, I could see that it appears to have been generated using some sort of industrial publishing tool (probably from Adobe) but that they simply suppress underlines with:
i.e. all link underlines suppressed, and this does work for me (now). I had tried using text-decoration before (first thing that came up in a web search on the subject) but had put it in an enclosing <p> tag rather than in the <a> tag, possibly through misunderstanding an example I had found.
I like DiapDealer's idea of global suppression of underlines, and simply underlining explicitly, as it gives flexibility when you want underlines for things like endnotes.
that text-decoration was working as expected, and at one point in my experimentation did see my contents page without underlines (but with example code and other experiments above it in the file). On cleaning up the underlining has come back, so am now working with a short test file, similar to DiapDealer's, to try to understand what is going on (so far without success).
At the moment what I am seeing is that putting 'href="..."' in the <a> tag makes the underlining come back, even if text-decoration is set to "none" either in CSS or inline. Obviously not very useful. The "Bad Pharma" example does use hrefs, but with a referencing method I have not seen before, e.g.:
<a href="kindle:pos:fid:000A:off:0000000001" aid="1T14P">3 Bad Regulators</a>