Remove the cup and spring,slide the nutdown the wand, unscrew the ball joint using the 2 small flats with a spanner taking care not to lose or damage the tiny ‘O’ ring.
Slide the nut off the wand and carefully remove the (misplaced) ‘O’ ring and check for damage / nicks.
Wipe out the nut,lubricate and refit ‘O’ ring and reassemble. That spring does look rough on the end.