I also notice that the pressure at the expansion vessel is quite low -
about 0.5 bar.

Do you mean the water pressure when filled, or the pre-charge pressure?

If the former, increase the pressure to 1 bar* by connecting and opening up
the filling loop for a few seconds. If this doesn't fix it, then you may
have an airlock, or a control system problem.

Alternatively, you may have a gravity circulated system, which just doesn't
work. This type of system is very unlikely on a sealed system.

Control system problems might be:

1. Failed cylinder thermostat.
2. Cylinder thermostat set very low.
3. Failed zone valve.
4. Failed programmer.
5. Programmer not set to heat.
6. Wiring damaged
7. Balance valve closed off.

Christian.

* If your house has more than 3 storeys, a more careful calculation must be
done.