RobertD850T5

I have recently bought a spanking 850T5 estate(1995).It is a thing of subtle power and elegance...but wait,there is a small fly in the ointment.
the remote alarm/central locking will not work.
I have done the obvious and changed the sender unit batteries but it is dead as the dodo.
My local Volvo garage want £50 to diagnose the fault,but i'm as tight as they come and want to sort it myself.
do any of the good people in volvoland have any tips as to where to start tinkering?
all ideas gratefully received!!!!!
many thanks.
Crazy Rob.
850T5 20v