How to reset immobiliser on Ford Escort 1.8 Diesel?

I had a problem starting my Ford Escort 1.8 TD, M Reg (1993?). Assume it was air in fuel system so took battery off to charge. On reconnecting battery, the immobiliser wont work. It is one where you have to use a black fob with what looks like a hearing aid battery in it, situated to the right of the steering wheel, with a red light on it. Instead of flashing red, it is now solid red and there is a constant high pitched noise coming from somewhere inside the car. I guess this might have happened to others on changing a battery. Please help.