I actually got the answer to this on this forum but thought I’d post it more clearly for anyone else in the same position.

Thanks so much to the original posters.

I’ve just change from a sole trader to a limited company which took Amazon around 3 months to process. They finally did it, and told me (understandably) that they now needed limited company bank details.

I must have tried 20 times to enter the IBAN of my old account which could never be verified and I assumed it was an Amazon system issue.

Anyway, long story short-Barclays for some reason have changed their IBANs. The great advice from the other poster was to look out an old statement (which I managed to get on the Barclays app) and use that IBAN instead. Essentially they have changed from the letters being BARC to BUKB, so if you’re trying to use an IBAN for an account older than a year or so, you need the old BARC one instead of BUKB.

Hope this helps someone else as much as it helped me.