If it’s 1.1/4”: https://www.screwfix.com/p/mcalpine-s23u-multifit-blanking-plug-white-32mm/73053 or one of these and possibly a coupling if you have solvent cement https://www.toolstation.com/socket-...MI9MDJtKun6QIVwu7tCh3lrwz5EAQYASABEgKyV_D_BwE
You will need to cut off the white pipe ,leaving a 25mm stub of white pipe. Then clean the pipe stub outer diameter of any irregularities ,if need be. Then fit a McAlpine compression blanking plug...see Screwfix code 96079 if pipe is 40 mm or if 32 mm code 73053
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