ferroli mondena boiler?

when the domestic hot water tap is turned on, the water runs hot for a few seconds then runs cold, but when the water is turned on slowly the water will run hot. part of the gas flow from the boiler to the gas meter pipework is in 15mm, could this be part of the problem. also on the central heating side the last radiator will not heat up. the boiler is a ferroli mondena.there are no thermostatic valves on the radiators. thanks