2006 S-Max 2.0tdci duratec overheating

Car has been overheating recently and I’ve found the fan isn’t working. I’ve checked the the ‘pink’ wire going into the fan control unit and it measures between 3.4 - 4.6v , so going off what I’ve read this should be ok, however, I replaced the control unit just to make sure but hasn’t made any difference.

The bottom and then the top pipe do get very hot so the thermostat is obviously working but after a short drive especially if a bit spirited the temp on the gauge shoots right up and the car power gets limited, I’ve noticed when this happens the expansion tank has some sort of safety valve and it blows a load of hot water out. 

I’ve also noticed ‘worryingly’ that the water in the expansion tank is a murky almost black colour , I’ve topped it up a few times but will need to flush to see if the murky water continues.

I’ve had the garage look and there’s no signs of a head gasket issue, though power isn’t what it should be at the moment “see my other post” they don’t believe the two are connected. I had the AA out the other day to replace a snapped aux belt and he said the overheating might be down to the water pump but that was before we knew about the fan.

Is there anything else that could stop the fan working like the engine temp sensor for instance? The fan itself does work as I’ve rigged 12v to it and works fine. Would the Pink wire measure 4v if it were damaged? 

Thanks for any help.

T