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If it helps, G. rivale has sepals (just behind the petals) of purple, rather than G. urbanum's green ones: and the really easy distinction is that rivale is more of a pinky-purple colour in the petals, whereas urbanum (which is everywhere round here, by the way) generally only comes in bright yellow.
Anecdotally I would also say that G. urbanum flowers have spreading petals, and tend to stand up quite straight, whereas G. rivale has a flower that is more of a bowl-shape, and they droop over on their stems.
Hope this helps!
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 54.17746, -4.50818
OS grid ref: SC363786