Hi,
I have also recently bought a house which had previously been rented and the tenants had done a bunk owing Npower. I phoned Npower customer services and they told me I had to send a fax, with proof that I owned the property (mortgage statement or house insurance, something like that) and meter readings (not relevant in your case obviously), and that it would be dealt with then. Apparently it can only be done by fax for some bizarre reason.
They then sent a revised bill to the previous tenants at the house, even though we all know they don't live there anymore!! and I'm still waiting for confirmation of an account in my name. But they used my meter readings to do the revised bill for the previous tenants so someone has worked from the fax.
The fax number is 0191 569 1353.
Good luck, sorry I couldn't be more help!