Sorry,but I think you lnow what I am going to say.
Although in the first instance the tenants ARE liable for the unpaid utility bills,if you cannot find/contact them then the payment of these bills falls to you as the Landlord/Freeholder.
IF you choose to ignore them (as I hope you won't) then the utility company would probably come after you as the soft option.
Once you have paid these bills, you could try and get redress in the Small Claims Court.However,if you don't know where the tenants have gone,or if they have insufficient funds to meet the courts judgement you are still in a fix.
I presume (from your question) that you didn't take a deposit from the tenant,or maybe you refunded it before the non~payment of the utility bills was apparent?
I suppose what I am saying is,pay the bills,and then if possible pursue the tenant.
Non payment of these bills would result not only in pusrsuit by a debt collecting agency,but also disconnection of supply.This would not bode well if you are trying to re~let the property to a new tenant.
I am sorry if this is not what you hoped to hear?