Solved it now, but after fitting the new blade I had to cut away some of the plastic from the mounting clip, in a brand new Bosch wiper, because otherwise it wouldn't sit square on the screen. Replaced like-for-like too, same size, same make! (18 inch)

Just thought I'd post in case anyone else is experiencing the same problem. It seems this time if year is when they split, (1) there's more rain and (2) they get brittle in the frost and (3) you mistakenly pull them off a frozen screen and any weakness now shows up!