Ok so managed to figure it out, and thought I'd post an update for anyone in a similar predicament...
Turns out I was using the wrong IP address in the Sky DMZ, and should have been using the WAN IP address stated within the 'Device Manager' tab (which took a little bit of time to update), so all now is working as expected.
@Netduma Support - can someone please now connect to my Netduma R1, as port 7777 is showing as open! 🙂