That wasn't cheap but it had to be done. The previous owner of this 44-year-old unit had fitted a water filtration unit to the sink. The spout for that had snapped off years ago and the end of the plastic pipe capped.
Last night I noticed some water on the kitchen floor but put it down to being splashes from hand washing of dishes.
Tonight there was visible dripping from the bottom of the cupboard doors under the sink, so I hunted around for an after-hours plumber, by that stage it was after 8 pm. I found a reputable emergency plumber in the area and he arrived about an hour later. He confirmed the leak was coming from where the water filter's plastic hose connected to yhe copper pipes. He gave me a quote to fit a new filter unit but I've become accustomed to the taste of the local water and just wanted the old thing removed and professionally capped off. The job was done within an hour, now I'mdealing with seven soggy towels. I'm going to give them a riinse and spin cycle in the front load washer to fix that before hanging them out to dry.