Domestic Hot Water is only tepid on Worcester Bosch 26CDi Xtra Combi

[PJG24]

CH works perfectly.
Fan is operating.
There are no blockages in the flue system.
The diverter valve is functioning, as is the microswitch (continuity tested)
The DHW sensor is working, as recently replaced.
The Overheat Thermostat has just been replaced.
I replaced the Control Board as per instructions in service manual.
Can anyone help me to resolve this problem? What would be the next step?

ps I am capable of replacing all components, just don't know which one as my manual is limited.

Very much appreciate any advice

PJG24

 

[J3000]

Could be a blockage in the plate heat exchanger, have you checked it? May need cleaning or replacing