Unwanted pre-payment meters

Other

I have now got three houses that have had pre payment meters put in because tenants have fallen into arears with payments.

One tenant asked my permission, the second tenant did it without my permission but gave the providers permission to enter and change the meters. The third tenant had left the property before the provider even changed the meter!! (was on outside wall) I could have told them the property was empty. She had given me over one months notice that she was leaving at the end of her 6 month contract. E-on didn't contact me as the home owner even though they had my address and telephone number!! I was left with a mouldy fridge and freezer as the electricity was cut off for a week before I realized.

Is it legal for providers of gas and electricity to do this without the home owners permission as they say it is?!?

They refuse to change the meters back unless I sign an agreement that I will be responsible for any future arears by future tenants and even want me to pay for the change of meter. As if I would do that!!

How can this be legal? Please tell me it is not, and what I can do to make them put normal meters in as my usual tenants are young professionals who will not want the added expense and nuisance of pre-payment meters.