diagnosing a blocked radiator??

[jamesheath]

I felt like I ought to update this thread.
I've filled the rad with paraffin for a couple of weeks, and got some interesting white sludge that settled out of the paraffin, and a little black sludge, though not much, however the rad remains mostly blocked - about a third is free flowing.
So it's time to give up and look for a replacement radiator for a re-core.
   
(this photo is of a hot rad, having cold fluid pumped through it - the left is flowing, the right not)