Have I f***ed it?

[VNA]

I think you have high resistance in the fan circuit and that's causing low voltage and low current at the fan.

Excellant post Dudeness. :thumbup

Thanks for the advice - I've just ordered some back probes for my multi-meter and will give this tests a try. Cheers

Yup all the stuff, everything, His Dudeness says, all makes sense.  In the meantime while you are waiting for your probs clean all the connectors and make sure they are nice and tight.  Wouldn't be at all surpised if it's just a bit of corrosion on the fan connector.
Very much doubt you've cooked it by the way - fingers crossed.