Thank you everyone who has reported login issues to me. The main common factors seem to be timescale (it seems to have started around end of October or beginning of November) and OS (one or two people on Mac OS or iOS seem to have had one-off problems but the persistent problems seem to be under Windows).
I’m afraid I am baffled as to what is going on, but am following up with tech support. If ever you cannot get into the site please email me and I will do my best to assist.
Apologies again for the inconvenience. This one is a real puzzle, and only seems to be affecting a few percent of people which makes it hard to pin down.