Why not fit the pump then arrange for the boiler to be serviced,
this way the necessary adjustments will be made by the engineer and the paperwork will be issued.
If questioned about the new addition "it's always been there" works.
You then get a boiler service as well as the paperwork.
That’s what I do.
We had a new boiler fitted by an “expert” after British Gas suddenly condemned it and quoted £3k for a replacement, I went for a local company. We had an open system and the “expert” connected it so the full hot water circulation went through the open header tank. The whole system gummed up with oxidised gunk. I sorted it and converted to a closed system then got BG back to service it. No problem.
