how do I fix the leak from the back of the DHW flow valve in my alpha 280E ?

I have a dribble of water coming from the back of the DHW flow valve in my alpha 280E combi boiler. I have taken the unit off, cleaned the limescale from it and re-attached several times but still the leak persists. It started leaking a little while ago and is worse when running the hot water. The system works ok it just leaks. Has anyone else had this problem or does anyone know of the usual site of the leak please. Can I buy a seal repair kit for this and the primary flow valve?