Any Guidance on a vaillant Turbo max 242 hot water?

I have found this site quite a usefull ome as have a Vaillant turbomax 242 Combi that like a lot on this site does not allways provide hot water but I do have a very good engineeer and we have manged like most people to track the problem down to the change over valve located between the pressure gauge and the on/off switches for when flick the contacts open manually with a hot tap running hot water is produced- understand its not the easiest job to change and a service kit ( vaillant B12?) but canany body shed any light on suppliy of parts for have tried Vaillant with no success- any info to me would be great