Thanks for all your assistance - I'm pretty sure my NanoVNA has a serious fault. A new one is on order. I checked the antenna with a SARK 110, and got exactly the result I expected, so that eliminates the antenna and feed as the problem. My NanoVNA is actually not the H model, but the vanilla one. Have upgraded it to what I believe is the latest firmware - v0.8.0. Development of this branch seems to have ceased in 2020, so time for an upgrade anyway. Have bought the latest 4" H model.
