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Det Ch Supt Fisher said that "there is no doubt that Dunleavy’s victims were let down by those who should have protected him".

But on the 20th anniversary of the murder, Det Ch Supt Suzanne Chow vowed the re-investigation would be "full and thorough".

Det Ch Supt Chow, who has been involved in other cold cases including the 1984 murder of Mary McLaughlin, said the re-investigation would take as long as was needed.

Ch Insp Stephen Wiggins said: "Filming in public is legal, however where this filming crosses the line into offences such as upskirting, stalking or harassment, it’s important that we don’t allow that behaviour."

Det Ch Supt Minnock told the BBC that 200 people had been returned to the UK this year as a result of these checkpoints, thought to be only a small fraction of those crossing the porous border illegally.