Overheating .

Now some will know that daughter number .....the one who's down south has had issues with her 54 plate M13 engined machine overheating ...usually in conjunction with a belt snapping .....however we believe now it's was a few issues together and the belts are sound , the alternator is doing its thing ....but on Friday coming oooop norf after many many miles of no issues , it overheated  
 Probably down to stop start Friday traffic , but it shouldn't . 
So after sorting the other daughters transfer box diagnosis I moved on to this one . Checked the radiator , not blocked and clear, no sludge , flushed it through anyway ..yep clean..belts fine ...new rad cap a few weeks ago .
 The only thing left is the viscous fan , I tried to stop it with a piece of card board but if kept going ... So am I correct in assuming ( yep an ass) that a new viscous coupling may be the answer ....There's nowt left in th cooling system to replace ...except the thermostat , but generally it's always works as it should and this time it was in solid traffic on M6 , so airflow issue.

I'd stick a new water pump on it. The fan not stopping is a good sign. It's when it stops easily that the viscous coupling has failed.

The cooling system can hold pressure? Just throwing this in as something to check.
ALso feel the radiator to ensure the heat transfer is good and uniform across the fins. I have had old radiators that flush ok but are obviously blocked when you "feel" or have corroded fins reducing efficiency.