You need to tell us what the fault is. If the locks have become difficult to turn with the key (as Ford locks do) then just squirt some Duck Oil (Duck Oil is now a Swarfega brand) into them. A squirt of Duck Oil will also stop your locks from freezing up in the Winter.
You would clean Cotswold stone using a product called Swarfega Brick and Masonry Cleaner. Wearing household gloves, mix the cleaner in some warm water and dip a plastic brush in the solution. Scrub the stone gently a wipe away any residue with a clean, damp cloth.