Thank you for the replies. You have put my mind at rest. As an added precaution here, where I am using my Sky router as little more than a modem to connect my main router a Netgear 7000 to the internet, I had allocated the the I.P. address of the Netgear as a DMZ so as to stop the Sky router firewall getting involved in any way. But I have decided to remove that DMZ setting from the Sky router, to allow both firewalls to catch any problems. I will see if it causes any other problems in doing that...............