Is the pump actually spinning? sounds like it is if rads are warming downstairs at all.

There are thermal sensors on hot water pipe and c/h pipe I think, also an overheat sensor. Could be the c/h one .. If you can find a manual, check for that.
other thing could be a faulty modulator valve .. thats a bit more pricey ...

But I think you need the manual for this one .. to follow the fault finding sequence.