Thank you 
Hooray!!!
I managed to take a “flasher” SD card, mount it on a Linux box and apply the patched kernel using tools/install_kernel.sh
The flasher then successfully writes to the target BBB internal SD
Robert, did you recommend working on a “non flashing” image to allow more customisation, in order to create a form of golden master? Installing packages and the like?
Thank you all so much for the help
Tris