Thanks both of you.

I checked tpl_header.php and tpl_main_page.php against originals and there are changes to do with my template in use compared to stock files from Zen Cart, but the core of the files are the same. I tried, just to test, dropping in the standard files without modifications & the error persists.

I also went through files looking for extra whitespace & couldn't find anything. The strange thing is, this site has been running for a long time & has quite a few plugins which work perfectly together. This is the first plugin I’ve used which fills up the log files, so it would seem if the error exists in one of my existing files, it doesn't affect any of the other plugins as the only file I ever get in logs is if a square payment is declined or fails.

I spent a few hours last night combing through files trying to find anything that could be issuing a header response before tpl_header.php, but I couldn't find anything. I'll just have to leave it disabled for now which is a shame.