Hi,
This is a case where really both companies are at fault, although Npower believe it or not are less at fault!
Basically British Gas should have sent Npower change of supplier details on your behalf, this is something that is legally required so NPower really can't do anything about changing you over. However NPower should also have been chasing this for you.
Phone up BG with your final reads etc and both of your meter serial nos. If they are still saying that there's nothing that they can do initiate a complaint as they are failing in their basic service. As soon as they contact Npower with this info it should take up to 28 days for the changeover to take place.
If your still getting no joy though contact Consumer Focus (The new energywatch).