Hmm. I may have an idea. I appear to have two versions of Arduino - 1.8.3 which I downloaded, then 1.8.8 which I got from the Windows Store. I updated the boards in 1.8.8 but I wonder if B4R is looking at the 1.8.3 version.
SOLVED. Uninstalled all versions of Arduino including the one from the windows store. Installed lastest version from Zip file, set up the file preferences for Arduino in the usual way, redirected the path in B4R and all ESP32 boards appeared.