Female Stag Beetle
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I tend to agree with your identification. This does look more like a greater stag beetle than a lesser.
Comparing this with the illustrations in Michael Chinery's Collins Field Guide to the Insects of Britain & Northern Europe (3rd edition), it looks as though the purple sheen on the elytra is typical of the greater stag whereas the elytra of the lesser stag are a matt black, which agrees with what I remember. The shapes of the thorax and head also appear more like those of the female greater stag illustrated.
Some better data on the size would help. In my experience female greater stag beetles are usually about 35 mm from the tail to the tips of the closed mandibles whereas lesser stags of both sexes are typically around 25 mm.
The lesser stag has just one tooth on the middle tibia, and the stag has three teeth, which you can just about see in the pic of the beetle in my hand.
The thorax and elytra of the lesser stag are matt black and heavily punctured with small punctures which contributes to the matt look of the insect.
The elytra of the stag beetle are unpunctured and shiny, varying in colour from black to chestnut brown. Size wise, female stags range from about 30mm upwards.
Tachinid Recording Scheme
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Thanks to both of you for your comments, which are much appreciated.
Lat/Lng: 51.3, -0.1
OS grid ref: TQ3468