Monotoma picipes
The hardest bit about identifying these was getting to the correct family. With 60x magnification I had not been able to see, or at least count, the correct number of tarsal segments, so eventually resorted to skimming through the images at the UK Beetle Recording website's families page until I found something similar. Once in the right ballpark getting it to species was relatively straightforward, the pair of basal foveae on the pronotum and the deep foveae on the head being distinctive. The only slight quibble was that one showed a very indistinct pair of shallow foveae in the front half of the pronotum, unlike the single central fovea shown in Duff, but these were not joined at all to the basal pair (as in quadrifovealata which also lacks the deep foveae on the head). Getting the foveae to show in photos is nigh-on impossible, my best efforts shown below.
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Monotoma picipes showing its head and pronotum (2 views), Wendling Beck Environment Project (Norfolk, UK), 11th September 2023
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second Monotoma picipes showing its head and pronotum, Wendling Beck Environment Project (Norfolk, UK), 11th September 2023
As is so often the way, the next time I encountered the species was much easier.
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Monotoma picipes showing its head and pronotum, Wendling Beck Environment Project (Norfolk, UK), 1st October 2023