Danny Kedwell converted two penalties for the Gills in quick succession early on, with Tom Flanagan being sent off for the foul that led to the second, and a Patrick Bamford brace then had the visitors level by half-time, before Danny Hollands nodded in the winner.
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A number of teenagers, aged 13 to 15, from Portsmouth, have been treated in hospital, after taking tablets believed to be diazepam.
Thursday Evening's Pollen Forecast
Thursday night's weather for the east of the region