Central locking for Ford Focus 02 plate?

I went to a garage last week to find out what is wrong with my central locking. They checked fuse and battery in key which all seems to be fine but the plastic covering around the key no longer works hence why I can not use my central locking (sensor from key to car). Have to open and lock manually with my key. I was quoted to get the plastic covering fixed that have the buttons on it for about £170!! Does anyone know a cheaper place to get this done?

Some one help me!!!!