I've now analysed my 20MB or port 30003 from yesterday. 239,885 lines of port output, and ALL lat/long values were wthin my normal reception range of:
54.6769N to 59.2829N
10.0957W to 00.8047E
Not a single value on the port 30003 output was outside my normal reception range. In my case, these spikes are definately caused by network aircraft.