Procedure to set up the control unit after replacing the battery

I have a faulty Viessmann Duomatic. It appears that the battery inside the control unit is dead. I have just had a heating engineer come and, after consulting with Viessmann over the phone, got my boiler working again by pressing a few buttons on the control unit. He told me the battery was 'tired' and warned me not to turn the power off until I had replaced the control unit (£450). I can easily change the battery but what buttons do I need to press to re-initialise the control unit or will it just work once the 3.6v battery has been replaced?