Overnight parking at Harwich Ferryport

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We are planning on doing our 1st European trip with our caravan in the summer school holidays.We are booked on the 9am ferry from Harwich to Hook of Holland, but we live approximately  a 4 hour drive from Harwich, so what we were thinking was drive up the
night before arriving at Harwich somewhere between 11pm and Midnight and hopefully park in the Ferryport. Does anyone know if this is possible or if not can anyone recommend a quiet lay by or car park close by to park in undisturbed overnight.