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Robin fledglings are certainly mottled, but patterns different. I'll post one for comparison. Possibly dunnock?
Revision - because it intrigued me, so I did some searching: not at all sure!
This is a fledgling Dunnock, as suspected by Roger. Dunnock can look very similar in shape to Robins (but ususually look slimmer). The juveniles of both are spotted but young Robins have pale buff spots while Dunnocks have dark grey/black spots/streaks as seen here.
Lat/Lng: 51.3, -2.2
OS grid ref: ST8759