What causes drain blockages outdoors?
If your outdoor drains are blocked, it's most likely due to a build-up of debris, dead leaves, and possibly tree roots. Autumn leaf-fall is the single biggest cause of gully blockages in North London — clear the leaves above the grate first if you can; sometimes that's all that's needed.
For deeper blockages, root intrusion is the next-most-common cause. Mature trees within 5–10 metres of clay drains will, eventually, find a way in through joints.