Cassiobury Park (above) is the principal public open space in Watford, comprising of over 190 acres and extends from the A412 Rickmansworth Road in the east to the Grand Union Canal in the west. The apparition of Lord Arthur Capel, 1st Baron Capel of Hadham, who was condemned in 1649 for his loyalty to Charles I and beheaded outside Westminster Hall, haunts the park. He is said to appear on the anniversary of his execution - 9th March.
For further information, please read Haunted Britain by Antony D. Hippisley Coxe.