What did your lease say? was he responsible for bills, if so, why is the electricity bill in your name?
Write to him at the address for him on the lease (you could also try to find an address on www.192.com) but send to the lease address unless you know he's definitely not there. State that your tenancy terminated on "x" date and despite an agreement made that the deposit would be retained in order to cover an outstanding electricty bill (if that was the case) you note that neither the electricty bil has been paid or the deposit refunded.
State that you would like him to send you a cheque for the full amount within the next 7 working days (Just to check, is there any other reason why you may not get the whole deposit back?) in order that you may settle the outstanding bill.
Ask that if there is any problem with this to contact you as a matter of urgency and provide contact details. If he is unwilling to comply for any reasons ask him to state his reasons in writing.
State that if you do not hear from him within the 7 days that you will send the letter by registered post.
If you don't hear then send it by registered post with a cover letter giving him another 7 days. State that if you do not hear from him then you wil take legal advice.
Adjust the letter according to the type of relationship you had with him and keep copies of everything.
Pay the bill whatever the outcome, it my be an expensive lesson. See if you can come to an agreement with your ex-flatmate about paying for it. Don't avoid the company, talk to them and see if you can work out a repayment play. Ignoring them will only make matters worse, affect your credit rating and possibly result in legal action.
You could also try the Citizen's Advice Bureau -
http://www.citizensadvice.org.uk/ or depending on where you're