If you already have a key then most high street locksmiths will be able to duplicate them. If you don't have the key then you'll need to get a mobile locksmith out to the car but they should be able to cut and programme a key to fit - though you will need to show them proof of ownership such as the V5C registration document. You don't say which model of Mercedes it is, and in 2004 some had a 'flip' style key and others a push-in 'smart key'. With Mercedes-Benz being a popular brand, most locksmiths will be able to supply them. You can also get a replacement key from the manufacturer itself, but this will be a more expensive option. Take your V5C to a Mercedes-Benz dealer and they should be able to order a replacement using the car's chassis number. The company is very good at maintaining parts supply for its older models.