Update 2! Gave up, re-started PC,here’s a paste of what I’ve just put up on the the dt github bug page.
OK…this is getting odd!
Downloaded 3.6.1
Four times it couldn’t complete the installation. Have lost the screenshot I took. Something about opening darktable.exe
Re-started PC.
Installation went ahead fine.
Went to open dt.
Little empty window fleetingly appeared as it usually does. Then nothing.
Tried again. Ditto. Looked in Task Manager and no sign of dt…
HELP!! 
Sorry to be a total bore.