There have been quite a few messages recently noting the age of many posters here, and basically accepting they can't physically do what they did a few years ago.
The kitchen tap needed replacement. It was probably 30 years old - a mono with individual controls. I couldn't squirm my way under the sink to see how the tap had been fitted (a rather awkward position and many pipes in the way). I took a couple of photos the best I could, and they revealed a thread with a nut (10 mm?) holding the tap onto the sink. The thread was corroded, and the nuts holding the tails didn't exactly look pristine. I sprayed the lot with WD40 the best I could, but couldn't see where it had gone.
So I emailed my plumber attaching the photos. He agreed that the corrosion didn't look good, but said they'd done this replacement many times. I bought a new tap at Screwfix, and the plumber came round yesterday. It took him around half an hour to get the old tap off, and a few minutes to fit the new one.
Just before he left, his comment confirmed I'd made the right decision: "I've removed hundreds of kitchen taps, but that's the first time I've even had to use an impact driver to loosen the nut holding it on."