How can I fix Firebird Combi 70 - losing pressure?

At the moment the dial is hovering on the danger zone. The water is fine - as is the central heating downstairs.
Upstairs the radiators are hot at the very bottom but cool/cold above that. I have bled them but there is no rush of air and no liquid comes out.
when the heating is first put on there is a sound coming from the upstairs radiatiors which sounds like water rushing. The sound eventually stops.
The boiler is about 2.5 years old.
Thank you for any help you can give.