You should be persuing this through the TDP scheme. The mediation service will be the ones to decide whether the amount deducted is reasonable. As far as I understand it the onus is on the landlord to prove that he sustained the loss that he is deducting and if he cannot prove this then you will get your money back. Also, if he has used the insurance based scheme, which if you have had your part deposit back from landlord it would seem he has, then he must now put the disputed £45 into the scheme for protection until the dispute is resolved. If you tell him that you intend to take this action he may decide its not worth the hasle and return it(!?)

On a Tenancy Deposit training seminar I attended, the speaker (legaly qualified) was confident that under abritation scheme of the TDS is was unlikely that a landlord could legitimately withhold part depsoit in respect of unpaid bills as he would not be able to show any loss on his part if the contract was between the tenant and the utility company and he was not party to that contract. Also unless there were opened bills left behind in the property how could he show that bills remained outstanding? If a utility company gave him information about an account held by the tenant they would be in breach of Data Protection Legislation and if he opened mail addressed to the tenant wouldn't he fall fowl of some Royal Mail regs or privacy laws?