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When my upstairs radiator in 1 room is on (very hot) then all the downstairs radiators are cold. However, when the upstairs radiator is off (via thermostat in inlet), all of the downstairs radiators are very warm. Has anyone had this type of problem before and can anyone suggest where I can start looking to diagnose the problem

sound like your heating flow is all going through the upstairs radiator. Check the outlet valves from the down stairs are fully open then throttle in the outlet from the upstairs radiator till you have a temperature drop over the radiator. I forget how much it should be but if the pipe is of a temp comfortable to handle it should be fine. You could try flushing out your system as it maybe a sludge restriction in your lower pipes making the upstairs route the easist one. Drain and flush out. I removed each of my radiators when I moved into the house and flushed them through with a hose, amazing how much gunk came out. refilled and retreated system.
You will probably have to try a few different adjustments to find the best settings for each radiator.

I have been told that sludge affects only my downstairs radiators because my manifolds feed them from upstairs (I have concrete floors downstairs ). The sludge is caused by air in the system causing the growth of algae--( you get this in diesel fuel systems also, so you get system cleaners for adding to your tank before your MoT to clean your pipes and injectors---gives a cleaner exhaust.)
Air gets into central heating systems thro' leaking valve glands or other couplings, or in my case thro' permeable 10-year-old plastic flexi-plumbing, which I thought was a great idea at the time I had the system installed--less yorkshire couplings. A learned plumber who I trust because we once worked together, tells me that I have been lucky to get 10 years service without major blockage. I had to dump one large radiator ~6 years ago, and now have 3 blocked downstairs radiators, and am told that I must spend ~£1000-1500 to replace the small-bore pipes to and from the radietors. I am told that modern double-wall flexi-pipes will be the answer to the problem;--BUT FOR HOW LONG!!???-------Can anyone advise, or should I rip up the floorboards, and put in 15 mm copper.?

Can anyone confirm this ???
I just moved house and I have this problem !!
Radiators warm upstairs but all cold downstairs.
wouldn't a proffessional power flush fix this ??
my system has the 10 mm MicroBore pipework

A professional powerflush on a copper microbore system seems worthwhile, costing about £600. I am told that old boilers are a bit fragile, when scoured with these chemicals, but in my case, my old plastic pipes will need repeat cleaning too often.
Cure all air leaks, and have your corrosion inhibitor strength checked and topped-up at annual service time.
Getting a reliable local plumber to do annual services is well worthwhile, but check his references;-- a busy plumber does not need to invent tasks.
Microbore pipes of any material, is prone to downstairs sludging, so I am upgrading to 15 mm.
The old polypipe in my house is now banned from sale, and is replaced in the marketplace by PEX, eg Hep2o, and does not let air breath thro'.
I hope to be able to afford copper.

Is the pump on max speed setting ?

is the pump old ?

maybe the impeller has came away from the motor. or its not the pump isn't strong enough the force the water to downstairs.

The water when pumped will take the easiest route, eg upstairs rads.

If these are turned off there's no where for it go except downstairs. But the rads will opnly get warm, not hot.