Thanks to everyone for their comments and interest in this post.
I'm pleased to report that the original problem has now been sorted. Having damaged the compressor clutch coil I obtained a second-hand compressor for UKP50 (including carriage) from www.breakeryard.com/
My friendly local garage emptied and re-gassed the system for me and provided me with a photocopy of the electrical circuit diagram, which helped us to trace the fault to an intermittent compressor cut-out relay.
This relay has a violet case and is located in the engine fuse box adjacent to the ABS control module. By a strange coincidence the car developed an ABS fault just after fitting the new compressor, but this turned out to be a rear wheel speed sensor failure.
The electrical schematic may be useful to others (it covers Xantia models 1.6, 1.8, 2.0 and 1.9TD) so I have forwarded it to Clogzz for him to add to his photobucket collection.