ok, with a million thanks to patient Jackie for helping me out - I suspect there many be a bug with the standard Adobeviewer application in FW 15.3, as the font information is ignored. However, the
hacked Adobeviewer version does the trick (of course I had to edit the userstyle1.css file by adding the relevant @font-family code for the LexiaDaMa). So (but I have only just tried it), it looks like the Hacked Adobeviewer application works with FW15.3 too - I thought I had read somewhere this was not the case.
EDIT: but of course, with the Hacked version there is no need for the additional userstyle.css file in the root of the system - the css stylefile of the book itself suffice.