The long PP fits white-rumped better than western sandpiper. It looks long but don't find the bill excessively long for WRS.
Completely with you on this Mark.
I must admit to looking in on this thread a couple of times, but never had a proper look until now.
I can see the superficial resemblance to Dunlin, and also to Western, but those heavy dark shaft streaks on the 2nd generation scapulars should readily rule out Western Sandpiper. It's one of the key features we look at in Europe to separate Little Stints from Western and Semi-Ps-especially so with the rear-most feathers of L Stint. Both Western & Semi-P have fine pencil lines along the feather shafts of all of their new 'adult winter' type scapulars.
Once you consider the above, and have a good look at the bird with moulting 1cy White-rumped Sandpiper in mind, everything points to that species:
The long pp (sorry if you can't see it, but it's clearly there), pale-tipped greater coverts (and medians too?), heavy dark-centred juvenile scaps alongside the new grey ones (with heavy, black triangle-shaped shaft streaks), pattern on the lesser coverts (dark centres with contrasting pale fringes), face pattern, bill length and shape etc.
Compare the bird with this one if you need further convincing:
https://www.cbwps.org.uk/cbwpsword/tag/white-rumped-sandpiper-in-cornwall/
The gingery tones are there too (e.g on the fringes of some of the tertials, also on the edges of the greater coverts), though I think heavy (in camera?) processing has skewed the real colours somewhat. I could be wrong but isn't there also a hint of colour at the base of the lower mandible? ...Maybe a step too far...?