UK Landlord Forum / Life

Hi, I've received a letter from a debt collection agency wanting £380 for an unpaid energy bill to Eon. The dates check out as a 2 month period I had with no tenant so I'm happy I'm liable for the original energy bill. My question is since Eon made no attempt to contact me and no bill was sent to the let address for me (as far as I'm aware) am I liable for all the fees I'm sure are inflating the amount? I had the heating set for 1 he per day and showed the property twice so I can't see its used that much energy! I was thinking I'd phone Eon directly on Monday and try and sort it with them, has anyone had anything like this before?

Thanks