Thanks Jools, I will try that.
I can't help feeling, however, that there may be something else causing the problem. Their plug-in isn't in the system now and I've checked that the 10.4.9 combo updater has updated iSync to version 2.4.
Interestingly, I found a posting on the Salling site
http://www.salling.com/forums/viewtopic.php?p=19667 from someone who had a similar problem a few weeks ago (not iSync, but Bluetooth connection).
Jonas Salling had a suspicion that a recent OS X update caused problems on some systems by sending out notifications (to those apps listening) that the Bluetooth hardware has gone away, at which point the connection is dropped).
He suggested waiting to see what 10.4.9 did. Ironically, it seems to have broken iSync (for me at least)...