Hi again, Graham.
I'm not entirely certain why this has happened but I’ve just had a fiddle about with two of my passwords: my primary and one of the secondary accounts, and I can now send email again. It doesn’t really explain *why* my passwords stopped working or needed to be changed when I’d altered nothing, but for now (fingers very much crossed) they’re sending again.
For anyone else experiencing this issue out of the blue, do as Graham suggested and change your passwords. I think changing the primary one was key.
I hope it doesn’t happen again! Thanks for replying Graham.