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Looks like a sand wasp, stocking its larder. Not sure what the caterpillar is. It's presumably paralyzed, popped down the hole made by the wasp along with an egg, which will hatch, and then eat the caterpillar while still alive.
Would agree with Maus. The black legs are said to be a key separator from other sand-wasps.
Black legs are not a useful character when idenifying sand wasps - other species have all black legs too. There are 2 UK species of Ammophila, I would say this is A.sabulosa from the faint melallic sheen on the abdomen and the shape of the 3 submarginal cells on the wing.
Tachinid Recording Scheme
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Lat/Lng: 52.9689, 0.6162
OS grid ref: TF757444
In Titchwell Marsh RSPB Reserve