Internet wont work on one network but will off another one.

Been having issues lately where even though im connected to my usual virgin network no internet related things work.

Internet happily works if i connect to a crappy BT one though.

Ive tried restores, turning off firewall, thoroughly virus scanned (guessing defo not this as internet works on other network) I have flushed dns, released ip and renewed, reset the router and my comp and all manner of things.

Anyone got any clues? I use windows 7 and am quickly going mental!