It wont't matter whether you go to an agent or direct to the landlord, both should do a credit check - if they don't theyre asking for trouble and are very naive.
If your credit history isnt too good be upfront about it with the agent or landlord. They wont necessarily refuse you but may ask that you provide a guarantor (someone who would pass the credit check and would be willing to pay your debts if you defaulted on the agreement). Alternatively, you could offer to pay 6 months rent upfront - assuming you have the money.