I made an box gutter through the monopitch - through the back of the monopitch and appearing out the front between a pair of rafters. Put this in before the insulation so it sits below the tiles and is invisible from anywhere but inside the roof. Drains into a hopper. As I GRP'd the flat roof, I just continued the GRP straight into the box gutter. I stuck a loft hatch in the garage into the monopitch so I can access (with a bit of effort) should I need to. Below is a very crap drawing of the general jist