The motorcyclist is said to be aged in his early 20s and was wearing light grey tracksuit bottoms with a double light blue stripe down each leg, a black jacket and black gloves which had plastic caps on the knuckles. He was also wearing a green and silver crash helmet.
The bike he was riding is described as a black and green off-road machine with Evo-Tech written on the front shocks.
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Southern Health will launch a full investigation into an alleged 'abusive' voicemail sent to Dr Sara Ryan, mother of Connor Sparrowhawk.
Direct rail services between Southampton and Brighton will be disrupted until the end of this week.
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