and hello again from windows 10! i did need to do a little manual fixing up from the windows 8.1 side; with its bootlader installed, i could go into the windows 8.1 recovery command prompt and do bootrec /rebuildbcd to make it find and add windows 10 to the boot list ^^

as a "computers person" i do not know how to solve the problem correctly the first time, what i do have is an endless list of strange tactical maneuvers that might solve a problem and an ability to prioritise them