According to document H1 of the approved documents, for an unventilated waste pipe from a WC a fall of between 18 and 90mm/m is required. However, reading further on to page 10 section 1.20 states the following: If the figures in Table 2 and Diagram 3 are exceeded the branch pipe should be ventilated by a branch ventilating pipe to external air, to a
ventilating stack (ventilated branch system) or internally by use of an air admittance valve. So is 18mm the absolute minimum or for example is 12mm acceptable provided the pipe is ventilated? It appears a bit grey as there are no details in H1 for ventilated pipes.

I think the bit about "if the figures are exceeded" is referring to the maximium fall not the minimum.

My understanding is that if the fall is too steep the flow can suck the water out of the U-bend. Thats why the solution is to ventilate the pipe.

If you are under the minimum then the flow rate isn't fast enough to keep the pipe clean and it's prone to blockage.