Det Ch Insp Kevin Brown said: “We are in the very early stages of this investigation, but are treating this as a suspicious death.
“I want to reassure the community that this does not appear to be a random incident, we have a lot of officers conducting enquiries and at the scene and if anyone has any concerns, they can approach any of these officers.
“The scene watch on Belgrave Road is likely to stay in place for a number of hours. I apologise for the inconvenience to local residents but this has to remain in place whilst our investigation continues. I thank them for their patience."
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The RMT union has accused Southern of inventing figures which the company claims show record numbers of guards are going sick.
Thursday morning's weather for the west of the region
Thursday morning's weather for the east of the region