help with letting with agent

hope some one can help me with a letting question. I have a house to let my last tenant cost me over £3,500 in lost rent and cost the insurance company £14,000.00 in repairs to the building ,so know the house this back up for rent ,contact a letting agency to find a tenant only at a cost of £200 plus vat, and not to manged it ,they have found a young couple who visit the house and they came with mum and dad ,granny to see the house and like it and they want to rent it ,they have passed the credit checks , but but am i in the right mind that i would like a guarantor in place and ask the agent but she said that i would not need one???? due to they have passed the credit checks,but i feel that i would be happy if one was in place, due to they have never rented before and being a young couple (high risk)( so am too worried about letting ,are in the right to ask for a guarantor) many thanks for any help
peter:(grin):