Sounds like your "Domestic Hot Water Flow Temperature Sensor" has gone. When you buy these you get them in pairs (one also for Central Heating Flow Sensor as well, if the DHW (domestic hot water) one has gone the heating will follow)(Usually) they are easy to install just remember to drain eaither side heating/hot water when you install or you will end up spraying your PCB (expensive!)
here is a UK link for the spares for Ferroli Falcon 2 (you may find them cheaper else where?
http://www.hccfspares.com/Appliance_Type3.aspx?Manufacture_ID=31&Appliance_Type=GAS+BOILERS
Monkey :-)
Monkey
January 2008