shut off cold water mains, and drain central heating to below boiler height, regin make silicone grease or ask in heating spares place, make sure when refitting pipes to valve you replace all the washers and any O rings the boiler will fill when you re-instate the cold water and re-pressurise the heating, MAKE SURE YOU ISOLATE ELECTRICS TO BOILER, and have a bucket ready as it can be messy and the have a tendancy to air lock, water can cum when you least expect it
STE, April 2009