Thank you everyone for the various suggestions. In answer to the last, there are two keypads, for each of which there is an isolation switch, and the system has been tested with each indivdually.
I shall now have to try the long way to identify the fault, disconnecting everything in turn. Actually, what I will do is use a "binary chop", in other words disconnect half the detectors, for example, and then (if the fault is not one of those) the other half.