diagnosing a blocked radiator??

[jamesheath]

Thanks Rick - so has anyone had success flushing stubborn crud out of a radiator with chemicals?
I read a long post about the chemicals used, but I don't think anyone described how bad their rad was to start, and whether it actually improved.... (fernox or Holts seemed to come out on top).
It's been backflushed (in position) with a hose, and the resulting effluent was pretty much clean (I really was expecting a brown sludge!)