My volkera option boiler is only heating the upstairs radiators?

Same thing happened to me, Have you tried turning off all your upstairs rads at inlets and outlets and turning on all downstairs rads.
If downstairs come on then there is probably not enough water in your system and as hot stuff rises it is going upstairs first.....
If this is the case there will be a valve somewhere, possibly in your hotpress that may be closed and will allow you to fill up the circuit, and then close valve again may take 10mins, do not have boiler on when doing this.
Very simple to turn off upstairs rads, give it a go..

Doyler
January 2008