HOW CAN I FIX BRITANY COMBI SE 80?

DO I NEED AN ENGINEER IF MY COMBI SE 80 SHOWS 70 OVERHEATING DEFECT WITHOUT LOCKOUT. I HAVE NOT USED MY CENTRAL HEATING SYSTEM FOR MORE THAN A YEAR DUE TO HAVING OTHER HEATING APPLIANCES. SO TODAY I TURNED IT ON, AND IT NEEDED BLEEDING, SO I BLED THEM AND KNOW I CANT GET THEM TO GET HOT