Improvements in or relating to well abandonment and slot recovery

A method and apparatus for casing recovery in well abandonment uses a slip 52 anchored hydraulic jack 18 having a piston 38 moving a mandrel 30, a casing gripping spear 20 and a pressure drop sub 24. A downhole flow pulsing device 22 on the string superimposes a cyclic pressure on the fluid used to move the jack piston to oscillate the mandrel while pulling casing. The apparatus length 27 is such that fluid at a first throughbore 43 pressure plus the cyclic pressure of a shorter inner throughbore pathlength 17 between the pulsator and the piston and a second annulus 53 pressure plus the cyclic pressure, resulting from a longer outer pathlength 19 via the apparatus lower end and up the annulus to the piston, interfere to enhance or reduce oscillation with the path difference determining the degree of oscillation of the mandrel. The path difference is an even or odd number of half wavelengths. The pulsator may be a motor driven rotating plate pulsator.