Shower and bathroom hot water

hello, im afraid you didnt need to change any of the parts on your boiler by the sounds of it you have a faulty shower cartridge. what this means is when your operate the shower/bath tap the water doesnt mix as it should and the cold is overpowering the hot and backing it up so on ly cold water comes out. im guessing the reason this only happens at the shower/bath tap is because the pressures of the hot and cold are almost equal and as a result the cold doesnt have the power to back right up the hot pipe to any other outlets. i would suggest you call an engineer, not the guy who's started changing random boiler parts. if you were saying that you had intermittent (cold, hot,cold, hot,cold hot) hot water at EVERY outlet then that would point to scale in the boiler and possibly might require a new secondary heat exchanger, but first point of call would be to descale the existing! the diverter valve certainly did not need changing. moving on.......... call an engineer, get the engineer to isolate both the shower and bath taps then turn one on without the other, this will identify whether its the shower or the taps. if its the shower, have him identify your shower unit, order and fit a new cartridge and if possible install non return valves to hot and cold supplys. or have your bath taps changed as it may be mixing at the bath taps not the shower valve. i hope this helps nic