Growing on riverbank which is prone to flooding in winter.
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likely to be a planted variety rather than a species.
I'm sure you're right about its garden origin, Hazel, but I doubt very much it would have been deliberately planted in that particular spot - so close to the riverbank. As I mentioned in my original post it is on a bit of the river that is flooded more or less every year. This raises two questions in my mind 1) It must be a pretty hardy variety/species and 2) were the corms washed down river and left here after the water subsided. As I'm sure you know crocus's, generally, are hardy but most require well-drained soil. I was rather hoping someone could have made a stab at identifying the plant as it is such a pretty thing and quite different from the usual varieties/species we see.
... it might be Crocus chrysanthus 'Cream Beauty' which is described as being bright yellow on the inside, creamy yellow on the outside.
This one seems very similar to yours:
If you try the name in Google images, you get a lot of paler ones, and a few just as yellow as yours are: like all garden plants, and particularly bulbs, a lot of them are mis-labelled, or optimistically labelled, making it hard to know exactly what something really is.
But if you wanted to source this very pretty little crocus, I hope that this name is a start.
Sorry I'm so late (a year!) but I've not long joined iSspot and I'm working backwards. Maybe this will remind you about going out to your river again, to see if they survived?
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 55.599089, -2.436724
OS grid ref: NT725340