potterton suprima 40L hot water but no heating downstairs?

I have a Potterton Suprima 40L about 6 yr old. Had no problems with it over the past years. I have no central heating to the radiators for the past 2 weeks downstairs, the radiators work upstairs though (not sure if that just down to gravity and being closest to the hot water tank). the hot water is totally fine.
the boiler is constantly firing up but the light only lasts around 30secs before going out, when it does go out the status light on the boiler flashes green constantly until it tries again. i have bled the radiators, took 4 buckets out of one to get hotish water out of it but within 2 minutes the radiator was stone cold again!
Does ANYONE have any clue as to what I can do next before getting a call out??
any help much appreciated!