Thank you so much @lifelongfrugaleer @TheGriffle and everyone else who helped me navigate VERY
Either they have the worst site I ever used, or I am exceedingly thick.
In case it helps anyone else, as has now been pointed out upthread with screenshots, the box for the code is right at the bottom of the PAYMENT section, which would make sense if it weren’t colour coded to match the “pay in full” section, and below the point where you terminate the checkout process if paying by credit.
Once the code has been entered, the prices of the items go down and you can then SCROLL BACK UP to do BNPL.
DON’T DO WHAT I DID (clicking “add to account” on the BNPL section). If you do that, you never get to the code. You have to SCROLL DOWN FIRST.
Annoyingly for me they will not allow me to cancel my first order which was placed without the code. I have to wait for it to arrive next Friday, refund it, and then reorder, which means I will lose the Black Friday Quidco, and possibly lose the Black Friday discounts too.
Honestly they have been a bloody nightmare from start to finish.
I am VERY glad (haha - see what I did there?!) that it has helped some other people though 😊