One of a number of minibeasts found this morning
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I think this is the larval (catipillar) stage of the Swallowtailed Moth, they're partial to Ivy and Hawthorn and are known to arch into an upsidedown U shape on branches.
The other possibility that occurred to me was Feathered Thorn, Colotois pennaria. From this view it not possible to see the critical feature. Are there a pair of small fleshy spines close to the rear end - about the level of the dark mark that your picture shows.
Foodplant - or at least the plant it was found on - and approximate size might also help. I find Feathered thorn most commonly on sallow, birch and hazel (though it is by no means confined to these) whereas the Swallowtail moth, as Alaine says, is commonly on ivy, privet and hawthorn and I've rarely found it in this stage but have bred it from eggs. In my (aging) memory it also looks a little slimmer than yours.
That's a valid point, I've just looked up feathered thorn I wasn't familar with the species in its larval stage. It would be interesting to find out for sure. Would you happen to be the same Richard Dickson who appeared on Springwatch several years back for research on mites affecting bees?
Lat/Lng: 52.7, 1.3
OS grid ref: TG2720