I deleted everything and started from scratch, and this time it worked (it's compiling as I write - although it has yet to complete, it didn't crash out as before).
The reason the original compile failed was, I now discover, my mistake. In the line
fakeroot make-kpkg --initrd --append-to-version=-custom kernel_image kernel_headers
I misread your instruction that "After --append-to-version= you can write any string that helps you identify the kernel, but it must begin with a minus (-) and must not contain whitespace.
", and also removed the kernel_image and kernel_headers parameters when I edited the text of the append switch.
A little more care, and all is well!
Thanks for your help