I've tried this now and it worked.
Created cue file in Imgburn, saved it, opened it with Notepad, added lines for ISRC with my codes, used cue file to write cd.
I checked it by making an image file from the cd I'd just burned, then opening the .cue file created with notepad, and the ISRCs were there.
All good in the proverbial hood.
Thanks again to Lightning UK