I have had a new Coservatory built with box guttering fitted where it meets the corners and flank walls of the recess in the existing structure of the house into which it is built. The box guttering stays about 1" inch full of water and the fall is just towards the right-angled corner of the house instead of to the downpipe at the end. The installer says this is normal but dirt is accumulating and it needs quite a rainfall before any flow starts down the downpipe which at its head is proud of the guttering. Is this normal and if not who can I ask to advise and argue with the Installer? The gutter is inevitably difficult to clean but that would be acceptable if the levels were flat