It turned out there was a problem with the street cabinet and the modem, not sure exactly what he did or was said as i wasnt there. However i'm told he went off to the street cabinet multiple times, after he had sorted the problem there (something to do with low power / signal i believe) he found the modem would still not connect.
He then replaced the VMNG300 with a Superhub and the internet now works fine again. He basically left us with a double nat as the Superhub was on the "R19" firmware which didnt support modem mode...
. However i found a post on the Virgin forums which explained how to force the hub to upgrade, and managed to get it to upgrade to the R36 firmware.
Its now running in modem mode with my Asus RT-N16 and i have no problems getting the full 100mbit speed.
I guess time will tell if the Superhub is any good or not, however hopefully as its just been used as a modem it will be fine. 24 hours and no problems yet (touch wood)