OK, I've found my problem, and it may affect others as well.
The script /etc/e-smith/events/multipop-save/S55conf-multipop will only rebuild /etc/fetchmail.conf when the connection type is defined as 'dialup'.
So, if you've got a permenant connection, and don't have sme configured as 'dialup', then fetchmail will never get rebuilt.
Commenting out the line if ($conf{'AccessType'} eq "dialup") in the above script makes it work as I would expect, adds/changes/updates now result in appropriate changes to /etc/fetchmail.
I'm not sure of a 'good' fix for this...
Cheers -