Good news and bad news Peter: I managed to delete the "books" using calibre (they were never visible on the glo) but I can't be sure of exactly what it is I did that worked - so not much help for anyone else who experiences similar. I'll try and explain as best I can what I did though - in case something jumps out at you or someone else.
First some basic stuff: I tried a factory reset; I tried deleting the files through file explorer; I downloaded kobo's pc software and sync'd etc but it couldn't show me these hidden books either; and I tried deleting through calibre on both a mac and pc. Nothing worked at the start though eventually the calibre pc option did.
On the device driver I never had show previews ticked at any time. The rest is where it gets a bit iffy and I imagine the answer lies. On the mac, I didn't have show recommendations ticked when I first connected the glo, I think I did have show expired books ticked but can't be 100% sure. I did try with it ticked and not ticked and disconnecting and reconnecting the glow after changing the setting. None of that worked.
When I plugged it into the windows pc I inadvertently had show recommendations on. Turning it off however didn't get rid of any books. I also ticked and unticked show expired books. Normally I was making all these changes with the glo unplugged and then plugging it back in. The last time is the only time I was conscious of making a settings change and having calibre restart while the glow was plugged in, in this case I unticked "show expired books." After that (not necessarily because of that lol) I was able to remove the books using calibre.
There is of course the possibility the remove books option showed up earlier and I didn't pay enough attention by just seeing what I thought I was going to see after playing around with it all until I was sick of it but I'm pretty sure that wasn't the case.
Anyway, it's all good now.
So thanks for your help