Many thanks to those who responded to the previous thread, still got
problems though...

I refilled the Saunier Duvall boiler, closed off the filling and failsafe
valves, and bled the radiators again. The central heating pressure gauge on
the boiler is now hovering just a shade above the 1 mark.

I think there's some sort of pressure problem though, the water doesn't seem
to be getting moved around the system.
The radiator nearest the boiler gets up a lukewarm temperature, I can feel a
slight warmth from two others, and the radiators furthest away from the
boiler are stone cold. Unfortunately these two are the living room and
bedroom
When I bled the radiators a trickle of water oozed out, every other time
I've done it there's always been a bit more pressure and the water would jet
out with about the same force as a squirt from a water pistol.
The boiler itself is acting wierd. It's too quiet, there used to be a
rhythmic sound to it, hard to describe, but you could tell it was working,
now it's silent most of the time. The burners don't ignite for long either,
5 to 10 seconds every 15 minutes or so.
I've tried all the settings I can think of, from the summertime hot-water
only all the way to get-me-through-the-nuclear-winter full monty. Nothing
On max settings the temperature gauge rises to around 50 degrees C, when it
was working it used to get up to 70 degrees.

Is there anything more I can try ?