Part L is concerned with energy efficiency. systems must be cleaned in accordance with bs7593
Nowhere does it state in part L or 7593 how to powerflush or that a filter must be fitted.
Side note - its generally accepted a power flush device is the easiest way to clean, but you can use a hose on a tap if it cleans to the standard.
but nowhere does it state a powerflush must be done, it states cleaned thoroughly
And It is not mandatory to fit a filter either - even though some manufacturers may extend Guarantees or stipulate fitment.
The benchmarks ask what cleaner and inhibitor was used and if fitted what filter
In answer to above post - no it isnt and no it isnt and it isnt grey either
Download part L and 7593 and have a read lol