It may be that the upstairs radiators are using up all the available heat before the pumped radiator water reaches the downstairs rads...
Turn the upstairs rads fully off - if downstairs rads now start to warm up you know the likely cause.
You can balance the radiators by adjusting the lock shield valves (closing off the outlet valves a bit) particularly on the upstairs rads - see...
http://www.miketheboilerman.com/balancing.htm
You can increase the temperature of the water from the boiler a bit but not too much else the tap water may become too hot - you can sometimes speed up the circulating pump to get the water around the system a little quicker.
This answer is close to worthless because I have no specific expertise in heating systems - but it might work...
Good luck...
Peccavi, October 2009
