As a driveway/patio installer YOU ARE BEING SHAFTED
That job is no where deep enough, the top pic above that shows the slabs, those slabs are usually 50mm (2") thick and he is barely lower than them.
Its not essential, but we always lay a sheet of plastic under concrete, it stops it drying too fast and its stronger in the long run as the concrete cures better, next week is suppposed to be hot as well
Depth should be a minimum of 100mm (4") of concrete not including the hardcore base, he will try and say he did dig 100mm out but he is using 50mm hardcore and 50mm concrete which is not strong enough
He actually didnt need to put hardcore base in, but as its cheaper than concrete, thats why he used it,
reinforcing bars are also not essential, we rarely use them on domestic driveways
to check depth, get a length of wood to go across the area and measure down or get a length of string and 2 helpers, hold it onto old driveway and path, pull it tight and measure from string to the top of the base
An old dirty trick is they will leave it deeper around the edges so it looks like they dug it out properly, but it will be high in the middle, only way to check is mentioned above
edit, if he gets funny with you, mention you will check with trading standards, for them to check the job when its finished