All the radiators in my house have fairly suddenly developed a problem
with not getting hot (it's happened over the course of a week or so at
most). It's a sealed system and during some boiler trouble 2 years
ago, I had the system flushed at the recommendation of the boiler
installer (actually it turned out that the system didn't need
flushing ... grr). The water going to taps is very hot, but the water
going into the heating system is barely above body temperature feeling
the pipes coming away from the boiler. Does anyone have any
suggestions as to what might have happened?

Jon