DHW flow switch sticking/leaking?

I hope someone can give me some ideas? I have a Halstead Finest Gold and I am experiencing a problem with the heating switching on after demand for the H/W has ceased. The heating will come on without the clock being activated. I believe it may be the DHW flow switch as when I tap it the heating switches off as it should, there is also slight water seapage where the switch fits onto the main body and the clip used to secure it has perished.

My question is, to replace the valve the switch connects to I would presume I have to isolate the cold water feed and drain the radiaters thus relieving the pressure so I may undo the valve? Will I be able to clean and refit the old valve or as it leaks do I replace it? At this stage I am unsure if it just needs resealing? Also does anyone know the part number for the valve thing i.e the plunger bit which activates the microswitch.

I am crossing my fingers that I have explained it well enough for someone to offer there advice.

Thanks. Nick