Circulation issues

[steelerblue]

Apologies if this has cropped up on here before, as I'm sure it's a common problem for a plumber but as an enthusiastic diy'er I've run out of ideas.
I have a simple, vented one-pipe system with header tank and cylinder fed from a back boiler which dates back to when the house was built in 1967. Most of the pipework is 15mm. There are no TRV's, the only control I have is on or off.
The back boiler works fine, the hot water is very hot. The first two rads downstairs next to the boiler are very hot.
The rads upstairs and the remaining one downstairs (which is last on the loop) are stone cold.
I've drained the system. I've changed the pump. I've refilled. I've bled the radiators. I've drained the system again and refilled. I've bled the radiators again (nice clean water). I've shut down all the radiators except one and bled that one.
Hot water is getting up to the first floor and into the start of the loop but not circulating. I've managed to get the first rad upstairs warm, but because there's no circulation it just cools.
I understand the physics of dense cold water and lighter warm water and gravity but has anyone any ideas???

 

[hammers4spanner]

Actually thought about getting a plumber in???
just post your location and get professional in

 

[steelerblue]

Yes, THREE to be precise. They've all scratched their heads, said it's probably the pump, and arranged to come back and sort it.
None of them turned up nor answered my calls when I chased them up.

 

[hammers4spanner]

problem being could be anything so not easy to give answer without actually being there looking at it so to speak , ask few friends to recomend one then .
Just by suggesting to try this that over net could end up causing all sorts of extra probs imho

 

[tamz]

Either the pump or a blockage.
It is unusual for the circuit of a 1 pipe to block.

 

[Nostrum]

Get in a plumber who is familiar with one piper's.

 

[steelerblue]

Okay thanks, will try someone else. Apologies if I don't sound enthusiastic, the last two I called in were recommended by friends/family.

 

[Barry98]

Put a post in thei need a plumber section , a member on here may be near you and able to help

 

[darrenangel]

I could be barking up the wrong tree here, but is the pump on the right way round, as in circulating the water in the right direction

 

[Nostrum]

Woof woof