The invention relates to a method of conducting a perf wash cement (“P/W/C”) abandonment job in an offshore oil or gas well annulus, in particular the washing or cementing operation using a rotating head with nozzles dispensing wash fluid or cement at pressure. A new design of bottom hole assembly is proposed in which the cementing tool has a relatively large diameter in order to optimize pressure whilst the wash tool has a relatively small diameter. The wash process, for a number of reasons, appears to be less sensitive to tool diameter and making the wash tool smaller reduces the overall risk of stuck pipe.