Yep, it actually started happening just before the great migration. I raised a ticket, it was escalated with their backbone suppliers and, after a few days, they came back and said that it appeared to be fixed, but if it happened again, I should switch my outgoing mail server to an alternative server address (which they supplied). However, they problem did indeed seem to be fixed so I stuck with my original mail server address.
As you have seen, it is now reoccurring (only with btinternet addresses)....but, like a numpty, I didn't record the alternative server address from my ticket, and the old ticket data hasn't yet been migrated to the new ticket system
Will give them a ring and see if I can get them to find the old ticket, with its new server address. In the meantime, I suggest everyone affected should raise tickets just to make sure that they know about it.
Can't blame a support desk if no-one tells them there is a problem