If an extra pump is needed for a conservatory heater, then are there
small pumps available? The standard CH pumps seem too large to me.

A zone valve would be fine instead. You can zone off with individual pumps
or with zone valves. Pump zoning also requires one way valves which are hard
to find in a quiet configuration. I much prefer valve zoning, particularly
if the main pump is a good one, such as a Grundfos Alpha.

Christian.