How do I change a diverter valve on worcester bosch 28cdi?

Ive tried numerous times to get a plumber out to change a diverter valve. They only seem interested in larger jobs so I`m gunna have a go myself because Im fed up with having no hot water..........Do I need to coat the threads in anything on assy? If so what?
Do I have to add an inhibiter to the system if so how? Are there any things that can catch me out? as Im doing this with no prior knowledge.