Again, many thanks for your speedy response and support, it's very much appreciated!
To answer your questions;
I do have just 1 i2c-dev.h (I did use "find" to search the entire file-system), ls -l
-rw-r--r-- 1 root root 2612 Dec 3 2018 /usr/include/linux/i2c-dev.h
Interestingly, and I think related, is this post, https://raspberrypi.stackexchange.com/questions/37689/how-do-i-use-the-i2c-bus, As can be seen from the "ls -l", i2c-dev.h with "Raspbian Buster" is 2612 in size, the download you suggest is 11507!
-rw-r--r-- 1 pi pi 11507 Aug 6 03:49 i2c-dev.h
$ dpkg -l libi2c-dev
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
ii libi2c-dev 4.1-1 armhf userspace I2C programming library development files
I followed your re-compile instructions, and THAT WORKED! many thanks.
I have a suspicion / intuition, that this issue is related to the "Rasbian Buster" build / install, PLEASE let us know what you find?
Again, many thanks!