Thanks for all the feedback - I'll have to have a good look all this at the weekend. What little I did find out is that in Mandrake (9.2 and possibly others) the compose key is the AltGr key.
The command
xmodmap -pke
dumps a listing of all the keycodes and the results of using alone, with shift, with compose and compose+shift.
I directed the output into a file for study so for example I have:
keycode 10 = 1 exclam onesuperior exclamdown
which gives (if you can see them!) the following four characters: 1 ! ¹ ¡
or: keycode 47 = semicolon colon dead_acute dead_doubleacute giving é for < compose ; e > which is different from Graham's example.
However I have for the first time counted how many keys are on the keyboard - 102 - and will now try editing the XFConfig-4 file (which is set for pc105 - I've never really thought about it before), restarting X and starting again.
Syd