The engineer from Virgin who installed my new router 3 weeks ago suggested speedtest.net isn't reliable as there isn't many hosts in the UK who host a server which supports up to up to 100mb. According to the engineer, Preston is the best server for people to use when testing their 100mb connection.
I'm just repeating what I was told. I have no idea if it's the truth. I'm not really an internet guy in these forums, I tend to post over in the TV section
