Have I f***ed it?

[His Dudeness]

I'm just guessing using a bit of basic circuit theory but hopefully it's correct info and he sorts his bike. Another possible cause would be high resistance in the fan itself. In that case you would be getting the full 12V at the fan connector when the fan is spinning but it would be turning slower than usual. To check that you would unplug the fan and put your meter to resistance. There should be a resistance spec for the fan in the manual.