This is caused by the cold water passing through the boiler, being heated, and then depositing limescale on the heat exchanger.
The best way to solve this problem is to replace the PHE, but this can be expensive.
The other option is to have the PHE flushed out.
This involves a special pump/flushing sytem and some quality descaling chemical.
This particular boiler is now running like new again, and another happy customer has been created!