have you checked that there is no air in the system by bleeding the valves.
if that doesnt work, check for a blockage (air or sediment) by switching off all the rads and switch them back on one at a time , bleeding as you go which will increase the pressure upstairs and possibly shift it.
if the system is fed by a seperate mini water tank in the attic, theres probably enough water in it as it would refill automatically, but if its a pressurised system, and was lacking water, it doesnt refill on its own. for that theres a valve in the hot press probably with a pressure gauge on it
.if you have one of those, open it up before bleeding the rads. if theres water flowing, it will probably make a fairly noticable noise
if downstairs is heating up, the pump is probably ok but if not,
a new pump costs 55 (heatmerchants brand)-85(grundfos brand) euros from heatmerchants, not hundreds. swapping it is not a big deal
switch the power off at the mains
tie up the ball valve in the attic if you have one
drain out the ch system outside at boiler (if its oil)
undo 2 pump nuts with a big stilson wrench
clip 3 wires into the new pump (same as a 3 pin plug)
refill the system
allow maybe 2 hours for this i8n total of which 5 minutes is for changing the pump