Pretty much comatose on the carpet a couple of weeks ago. Now surely, this one must be a blow fly! But which one? Very black from every angle, but it was in the process of dying - dead the next morning.
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Try Protophormia terraenovae.
Lat/Lng: 51.072, -0.69
OS grid ref: SU918311
In my house, in mixed broadleaved woodland, altitude 238m, oak, birch, chestnut and willow nearby.