Shower fall to waste - advice needed

I am currently building. My private building inspector has noted that the fall to waste in one of the showers is inadequate. The builder has refute that charge and is willing to put in writing that the fall has been checked and is fine.

I checked the levels with a spirit level. 2 out of 4 corners seem fairly level to me (ie minimal fall) but the builder insists that it is fine. I also tested the water draining and it is definitely draining albeit at a slower rate compared to the other shower.

The builder thinks that since it is draining, it is not worth re-doing the floor tiles, potentially disrupting waterproofing etc for just that little improvement in the draining times. He also mentioned that there are no fixed rules for draining time and this can be quite subjective and my building inspector obviously has a strict criteria for it.

So I am at a point where I either accept the builder's work and hope all will be fine. Or take it further to the department of fair trading as we have reached a disagreement and get them to investigate the issue as a third party.

Any advice?

What is the fall?

Get something straight, I.e. a level or timber.

Sit it from one end of the shower floor to the other end.

What is the height difference (gap) over the waste? What is the distance from the edge to the waste?

Fall should be 1:80 in the shower area.