not justified deduction from apartment deposit

After moving from my previous apartment, the landlord has deducted the cost of boiler maintenance from the deposit, but has never shown me a copy of the receipt. I have asked several times to see proof that the check has been done, but the landlord refuses to provide it. Is it not my right to have a receipt for something I have paid for? Who can I contact to convince the landlord to show me the receipt? Thank you for any information provided!