As a matter of law, a payment of rent made by mistake must be repaid by the landlord. But you must sue the landlord, not the letting agent.
As to how to find the landlord's address, this is dead easy: sue him at the property being rented! Since he owns it, legal notices can be addressed to him there.
You might also read this thread, which includes additional information about finding a landlord's address:
http://www.consumeractiongroup.co.uk...eductions.html
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