NO hot water in one tap , but in others ( on and off)

[HughDC]

Alpha 240E Gas COmbi,

There was no hot water (intermittently) in one tap, but was thers. The flow from the tap is fine ( in fact fast

I checked and saw that the pin that comes out to move the switch to turn on gas ( Is this on the diverter valve) did not come out as far as the taps that did work.
I turned of water dismantled by removing the brass cover to reveal pin on spring + diaphragm. I cleaned the pin , spring and various holes all of which had some carbon, and some hardness on.
This did not solve the problem.

I put as a temp measure a small pad on the the switch to make the smaller movement, switch on the gas but now the temp of water is low to all taps
Do I simply need to buy and replace the diaphragm, (if so part number ?) Or is this a bigger job ?

thanks