Message notification not working

Recently message notifications have stopped working but I don't think the timing coincides with any upgrades. I'm using iOS 11.4.1 at the moment.

Notifications for Messages is turned on. I've checked that nothing had changed the original setting. I also tried changing notification options to see if it had any effect but it doesn't.

When a new Message arrives, if I have sound turned on there is an audible notification but nothing appears on screen. If the phone is unlocked (whether it was locked or unlocked when the message arrived) there is no number on the Messages icon so I have no way of knowing a message has arrived.

Has anyone come across this and know how to fix it?