A couple of days later I noticed a very similar abnormality in a starling at Blyth. Like the great tit, it appeared otherwise normal.
I have reported the great tit to the Big Garden Beak Watch, the BTO's survey of beak deformities (the starling isn't eligible as it wasn't in a garden). In their results the great tit was the fourth most commonly affected bird after rook, blue tit and blackbird. Starling was sixth after jackdaw. Their data suggest there might be a geographical cluster of affected cases around Newcastle upon Tyne but numbers aren't big enough to confirm this.