potterton diverter valve

Here is my problem
I have a potterton performer 30HE.
Recently when the hot water tap is turned on the boiler takes some time to fire up this then got to the point of no hot water at all.
I had a look at the diverter valve and the pin on the front of the valve is slow or hardly moving and only moves if the hot water flow is turned up to full.
I took the diaphragm out thinking this may be the problem but no holes/splits could be seen so replaced after cleaning components and applying silicone grease to moving parts.
Problem still the same (only at maximum water flow will the pin operate the micro switch)
If I operate the micro switch manually the pin on the top of the diverter valve comes up no problem so is it a case of a complete new valve or can the be repaired ?
Many thanks

Did you change the diaphragm

No I didn’t change it I took it out to try to confirm if it had split
Before ordering a new one.
I couldn’t see any damage so put the old one back

Take out & clean the sensor pipes which go over to the other hydraulic block. Then remove the 24mm plug and remove/clean the bits in there - filter/flow restrictor stuff.

Diaphragms cn stretch, it may not be holed but itprobably needs replacing.