Colin & Bruce thanks for your comments.
My engine did not overheat, it stopped about 20-30 seconds after the belt broke and the temperature gauge was "in the middle" ...quite normal after it stopped.
As I understand it there are two fans ..... one works off the temperature gauge, the other off the temperature sensor via the CPU. I would imagine these are activated by relays, so I will get the garage to check those. The most logical answer I think is that a wire was damaged when the belt was flailing, so it will be a case of finding that?
So at the moment I am going to ask the garage to check the :
wire connections for breaks/shorts/continuity
Relays
Water system for blockages
See what temperature the CPU is reading
If they have no luck finding the cause, as a last resort I will probably install or suggest that they install a manual switch so that I can turn the fan off or on. I am happy to monitor the temperature gauge and flick the switch if it is needed.
(I know that the instrument temperature gauge worked .... when going down long hills I usually slip into neutral and drift down at around 60-65mph on tickover. Sometimes for a few minutes. The gauge drops one or two needle widths when I do this. I know this may be frowned upon by some but I have always done it whether in car or motorhome ... a hangover from student days when I had no money for petrol!)
Jeremy