I have spoken to my SS and while on site he double checked the internal plumbing in the slab. He confirmed that all was okay.
He did agree that there was one less pipe that was on the plans but that was because it was not required. The pipe is for the H&C water and the electricity conduit and apparently they can both fit in one so they don't bother with two. he assures me that this is the same as what was done on the other houses (there are a few the same in our estate).
Although I would prefer that things are as per the plans, I don't have the expertise so I accept that this is correct.
Now back to admiring the slab and waiting for the frame to start next week.