We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "JoeJoe" saying
something like:

Problem is that the radiators seem to be getting very cold very quickly as
soon as the boiler is switched off. My suspicion is that the cause could be
because some of the pipes (copper) hat feed the downstairs radiators are not
insulated. I have a 1m high space under the house, where the pipes are
located, and it is freezing cold and drafty there.

Does it make sense? Will it be a good idea to insulate the pipes? It
shouldn't be a very big job.

The underhouse pipes are obviously losing heat - and if the pump is
continuing to run after the boiler shuts down, then the colder water
from the pipes is entering the radiators and cooling them down. Even if
the pump isn't running, the next time it starts, it's shoving cold water
into the rads before the hot stuff (which has lost some heat to the
underfloor) arrives.

So yes; insulate, insulate, insulate.
--

Dave