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.

Steve Wedlock
March 2005