There is a man, in UK, who used to work for Ilford commissioning these systems for them while they were a current product. He still does that, and he is able to fix those, or at least, I would hope, to give you some advice. I did not ask for his permission to post his name online, so let me ask him, over email, and if he is good with that, I'd happily share the contact.

It might be cheaper, though, to get a spare power supply than to ship to and from UK.

By the way, you say that you have a fully spare system, does that include all components, 500S supply, 500H head, and 500C controller?

Probe should make no difference. Fan should be coming on and off, indeed, controller instigates that operation.