Before you can begin to deal with the arrears, first you have to agree that money is due and the amount is correct.
Ask the landlord to provide a full rent statement, look at your rent receipts, bank statements and your rent card if you have one.
If you get housing payments paid direct from your benefits, the benefits office can supply a statement of what has been paid to the landlord.
This information will help you and your landlord to match up and agree on the rent arrears amount.