Why are my radiators cold??!! HELP!!

[Teuch]

We have oil central heating, controlled through an 'aga'-type cooker and radiators.

The downstairs radiators are not heating - I have taken the wee funny key to them and been scooshed with the obligatory dirty water, so the radiators themselves not needing bled...

Boiler is upstairs and the radiators up there seem to be getting warm.

grrrr...bloody freezing here!