Who covers the sycamore leaves in this sticky stuff? Might the little insects on the underside of the leaves be responsible?
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"Aphids will suck the sap from Sycamore. Heavy infestations deposit honeydew on lower leaves and objects beneath the tree, such as cars and sidewalks. These infestations usually do no real harm to the tree but the sticky honeydew and black sooty mold below can be annoying."
You could re-classify the observation under "invertebrates", and one of the experts may know the most common culprits.
Lat/Lng: 51.4011, -0.1823
OS grid ref: TQ265684