Well, this was new.
I can see the cutter, it’s not missing, so it’s stuck. I look at my extruder and the area top the bottom left (where the cutter resides) looks “wrong” somehow. I remove the extruder.
The reason it looked wrong was the self adhesive label on the front of the extruder was curling up. The reason it was curling up is because there is a hidden magnet behind it; in conjunction with the magnets on the cutter it provides a sort of “sprung suspension” (you have to be careful which way round you fit the magnet on the extruder or it sticks in a permanently “closed” position"; when it’s the right way around, you should be able to push it in (closed), and the magnets will make it spring back (open).
It is ridiculous that al that is holding it in place is the self adhesive label. The adhesive had softened due to the heat of printing and it simply wasn’t able to hold the magnet in place. I’ve applied a small piece of electrical tape underneath the label in the hope that this will fix the problem.
Thought I’d share in case it helps.