Yes I did, an external WD drive connected via USB.
I never did try disconnecting it as came upon the PRAM (NVRAM?) flash and that worked as a solution that allowed me to regain control of the mac.
But thanks for the idea.
Boot Camp and Win 10 were eventually installed successfully after I deleted the Boot Camp partition, re-downloaded everything again and went through the entire process again, from scratch. All worked well this time (had the WD HD still connected throughout) an everything works except the Apple wireless keyboard.
Have resigned myself to revert to a wired keyboard connection as Apple support says the only way they can see (after some other troubleshooting) that might resolve that particular remaining issue is to repeat the wipe the boot camp partition, re-download everything again (onto a never-before-used stick), and go through the reinstall process again. Haven't been able to confirm though that Win10 or programs already loaded with key codes will be able to be reloaded onto essentially a "new computer" so haven't taken this last step to allow use of the wireless keyboard.
c'est la vie.