Ok. So I'm still having no luck whatsoever. AM about to reach the point of giving up to be honest.
Only thing I haven't yet tried is fixing the IP address of the N300 as per below.
Just two other points on my original Gigaset N300 setup:
On my router I fixed the IP address of the N300 basestation by adding its MAC address to a BIND list.
Also noted that my router (which does not have an FXS port) has this set by default:
Enable the First Priority for VoiP SIP/RTP
SIP UDP Port 5060
Also, I don't have any sort of priority settings enabled but not sure if that will be key.
One thing I have noticed is that whenever I enter the SIP Password into the N300 - when I go back into the settings the number of *** marks is not as many characters as in the password. Hence I'm not sure if something odd is going on here or whether that's just a quirk of the N300 interface