>is it possible to change the diaphragm within the diverter valve
yes
>is it best to change the complete diverter valve?
why when it's only diaphragm thats faulty.
>what is the average cost of a diaphragm?
depending on model about £12.
>the cost of the diverter valve
depending on model average £80
hi-spec plumbing, August 2007