Some observations on the content of this traffic;
- it seems to be initiated from my PC rather than my router
- whatever is doing it seems to be checking every port in turn ad infinitum.
e.g.
TCP, Src Port: 4546, Dst Port 80, seq 653, Ack: 756, Len: 0
TCP, Src Port: 4547, Dst Port 80, seq...