Why though? When Hartwell have given us a letter saying they have informed the relevant company that we are moving in? Why should I be liable to pick up a bill from a company that I wouldn't otherwise know about?
As you were living there using Energy whilst SE were the supplier. npower wouldn't come into this until 25th June, when your new contract/rates come into effect.
The Agent has caused all of this in my opinion, it's very sloppy of them. I suspect that they haven't notified SE of this 'change of tenant', so they are just producing a final bill as a result of you moving suppliers.
This is messy! I'd say that if the bill is addressed to your agent, then it's their battle/issue, not yours, but they may choose to seek some of the cost from you, given that you were using Energy (even though it's in their name) during 15th May and 25th June.
Have you had your first bill from npower yet? Is that correct...?




