Fully turn off all the upstairs rads and fully open the downstairs ones - see if downstairs rads get hot - if they don't then call a specialist in - if they do get hot then it may well be that the upstairs rads are getting the Hot Water first and using up all the heat before the flow arrives downstairs.
There are things you might be able to do:
Make the circulating pump work quicker (they sometimes have a speed adjustment)
Turn up the temperature control on your boiler up a notch so that the pumped water has more heat in it to share around the system - but not too much else the tap water will become too hot for safety.
Balance the radiators (close off the lock shield valves a bit on the upstairs rads and open them a bit on the downstairs ones) see...
http://www.miketheboilerman.com/balancing.htm
Good luck...
Peccavi, September 2009
