Just a further update. I have reset the configuration on the RUT955 back to factory defaults and setup no other services other than just the NMEA forwarding.
Using the Linux standard netcat command, so taking my application out of the loop, I get the following:-
uniport@raspberrypi:~$ nc -l -k -v -n 17051
Listening on [0.0.0.0] (family 0, port 17051)
Connection from 192.168.1.1 36493 received!
$GPRMC,095200.00,A,xxxx.xxxxxx,N,xxxxx.xxxxxx,E,0.0,0.0,050220,3.0,W,A*2E
Connection from 192.168.1.1 36494 received!
$GPRMC,095230.00,A,xxxx.xxxxxx,N,xxxxx.xxxxxx,E,0.0,0.0,050220,3.0,W,A*21
Connection from 192.168.1.1 36495 received!
$GPRMC,095300.00,A,xxxx.xxxxxx,N,xxxxx.xxxxxx,E,0.0,0.0,050220,3.0,W,A*2A
So to summarise, with the RUT955 back at factory defaults, no other services configured/enabled, "Contain connection" is ticked, NMEA forwarding to my device and using a Linux standard command to capture the data, I still get connection closed after each NMEA sentence.