no hot water or heating sabre he boiler?

Ive had my sabre he boiler serviced earlier this year and told the plumber keep havin to top it up as pressure keeps going down 5 wks after still doing same then no hot water same plumber came out said it was venturi pipe blocked has worked ok for 5wks but still having to top boiler up and today green ligjt flashing but as soon as i run hot water turns to red so now have no hot water or heating!! Any ideas anybody and should i get diff plumber is he ripping me off already paid out over £200