If it's a buy to rent then I think that some lenders state that the rent that you charge must be at least 150% of your mortgage payments. Beyond that you could have an estate agent sniff around the house to put a value on the rent payments.
How about asking estate agents to price it for you, like they would if for a sale? They will know what people are prepared to pay for a property like yours, and will be able to advise you in other areas too.
Just have a look in the paper for a comparible mate - If that fails, contact your local authority's rent officer, and they will be able to give you an amount.