I admit that my "straight edge" in the pics is just a random floor joist that I found lying around. I should have placed my spirit level on top to illustrate the point better. I demonstrated the problem to my customer with my 1800mm spirit level and it was just as bad, so much so that we had to check the window for level too. The dip was peaking at 25mm over 2.5 metres (if that makes sense) and fortunately appears to be quite localised as the rest of the 75sqm seems pretty good.
Yes, I am working with Weber on this job and will provide a review when the floor is completed. I have, also, offered to provide Weber with detailed feedback too.