V. old system, all parts seem ok, rads not heating up

Hi chaps
I've a fairly simple but oild system, Grundfos Selectric pump, potterton boiler in a single-floor flat with 6 rads - single flow pipe from pump feeds to 1 rad then splits two ways to feed 2 and 3 in the rest of the flat.
The boiler comes on ok and the pump seems to be running (light vibration from the body, and selecting different flow rates at the pump change the running sound) but the rads aren't heating up.
The flow pipe out of the pump is very hot, the flow pipe out of the valve is just as hot, then the flow pipe runs up the wall and the further you go along this pipe the less hot it is. I've bled all the rads but still no dice.
Any clues please?
Thanx
Malc

 

i am not a heating engineer or plumber, but you must have a water tank some where, is there any water in it?

 

All water is present and correct

 

is the pump on the highest (strongest) number? if you change it to a lower number does the heat not reach so far through the pipes?

 

Pump is on max (3) but I'll need to let the system cool down before setting it to 1 or 2 and doing a temperature check.
Cheers!

 

Ok - with pump on low flow setting, output pipe is cooler....
Airlock? But vent pipes only flowing water when bled, and rads the same.
Sludge?

 

OK I think I have it!
Checking up on the system it checks out to be a Honeywell Sundial Y-plan, and the 3-position zone valve looks like it's not operating properly.
When I move the lever to 'manual', the system flows hot but the motor in the valve isn't operating to switch the valve automatically.
More anon.........

 

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