Remeha avanta plus code 8 on display?

Hi there, I have a remeha avanta 30s. My hot water works fine but my central heating doesn't work at all. I've topped up the pressure (was way below 1 bar), and it seemed to work, with 0 on the display. But it isn't working ands now shows code 8 (no letter E), but this isn't in the manual...I've tried replacing the batteries in the control panel downstairs, and both panels seem to be working from what I can tell. Anyone out there know how to fix this pretty please?