Thanks Nicolas - you solved my problem!
As you said - turning off eBox's firewall made no difference...
Because I have only used mysql in xampp I was not aware of this default. BTW the my.cnf doesn't now contain 'skip networking' but has bind-address=127.0.0.1. I just changed this to bind-address=192.168.0.20 (my server's IP) and restarted mysql, and now get access to it remotely (I checked using telnet 192.168.0.20 3306, then connected using SqlYog from an XP machine).
I then turned the eBox firewall back on, and could still connect. I then removed mysql from the list of services in the eBox firewall, saved and was still able to connect! This is weird as I would have expected the eBox firewall to block access to port 3306 by default - obviously not!
This still leaves the problem of adding a php5-mysql to php (see earlier post above).
Anyway thanks again - at least I can write front ends to run on the LAN.