On Friday, 14 September 2018 08:27:46 UTC+1, Muddymike wrote:

So there is your answer. Your contractor should either increase the fall
or source a membrane that is suitable.

Is there sufficient height at the wall upstand to increase it?

Yes, I think so.

We'd only be looking for an additional 20mm-60mm which I am sure could be accommodated as there are 150mm lead flashings extending above the finished surface.