I was suspecting that that link would show your ISP as something other than UKONLINE.
The actual error you are getting is probably 550 Relay not permitted and that is because the ukonline SMTP server sees you as not coming from within its domain.
I had been suspecting that you may have been picking up an open wireless connection from one of your neighbours but if you are wired that is obviously not the case.
Both SKY and UKONLINE use the easynet backbone so somehow you are being issued the wrong IP range.
Are you sure the router has been flashed back to the netgear version. What is the firmware version reported as?