This is probably because the fob ran out of power and lost its coding before you changed the battery. So the key needs recoding to the car. Expensive at a Ford dealer. Some of these may be able to help:

Try a branch of Timpsons in the High Street, side street or in a Shopping Mall. No joy there, links to keys and locksmiths in this directory: www.honestjohn.co.uk/useful-websites/specialists/ The first two on the list apparently give very good, very fast and very cheap service. Auto Key Specialists Ltd., 948A Gt. West Rd, Brentford, London Middx TW8 9ES T: 0203 490 6001 Email: fobkeys@hotmail.co.uk also do a good job. Reader saved £56 on Dealer price for same key. The rear doors, indicators and rear wiper are a separate can-bus problem. Need a local 'car electrical specialist' (www.yell.com)