Am I responcible for tenants electric bills that they did not sign on for a account

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I let a house and told the tenants to sign on for the electric when they move in.
I have now got a bill from the electric company saying I as the owner are respocible for the bill,the electic company thought the house was empty and the bill was for standing charges.
This is going back 3 years and I have read the meter and I calculate the real bill is £2,500+.
I have never signed on or used any electric at the house and how they thought the house was empty I will never Know.
I know it used to be that you were only responcible if you had signed on.
There is no chance of the eletric company getting any money from the extenants as they left owing me a large amount of rent and I have no address although I knew were one worked 12 months ago.
Surely I am not responcible?
Any help/advise please.