Thank you for all the helpful ideas. I might have fixed it now. I took the stereo out in order to get to the immobiliser - helpful information from http://www.granada-and-scorpio-online.com
as well as from this forum.
I snipped some wires as per their diagram on the Website, and the car has not stalled since. In effect I have taken 2 out of the 3 circuits that the immobiliser stops out of the circuit.
It seems like a common problem on these immobilisers on 90's Ford Granadas because someone had already hacksawed through the mounting bracket, so the immobioliser had probably already been replaced.
Incidently, Ford quoted me £360 to do the job. If they had given it a wash as well, it would have more than doubled the value of the car!!