After discussing the mp3 thing with another Linux bod, I tried Amarok as that is what he was using to sync properly.
A quick play with it and decided it probably wasn't the approach I wanted, however in the process of following instructions somewhere online for using Amarok with the N95 I discovered the idea of telling the media player where the portable device is.
I wasn't expecting to have to do this as Windows software would generally figure it out for you. Anyway, /media/N95_storage_card was added to the Rythmbox config and away we go.
Still don't get syncing, but rather I connect the phone and do the following:
1. create a playlist of tracks I want on phone
2. select the playlist in the left pane of Rythmbox - this displays the mp3s on the right
3. highlight all the listed mp3s and drag on the device/N95 icon on the left pane
First time I did this a progress indicator started. I unplugged the phone after 100% but only got half an hour of music on my phone. I guess that progress indicator was a progress made in queing the tunes rather than actually transferring.
So while I haven't actually got a load of music on my phone yet, I do think I know how now :)