Good post Nik just discovered this as well.
Called back Virgin who tried to transfer me to someone who specialises in V+ but by accident they put me through to Telewest! Anyway the girl there saw what the problem was straight away. Note this only works if you have kept your original box and had it reinstalled in another part of the house. The V+ box must be the primary account and your old box the secondary, in my case the accounts were the wrong way round all she had to do was switch the accounts aroundthen i rebooted the V+ box (I didn't have to unplug the old box) and away we went. Still no catch up TV or on demand though, this is an on-going problem with the return signal so I am told. Big brownie points to Telewest.