Missing Rotation Lock

Which model iPad Air ? Depending upon the model (and your settings) it's either on the side switch or Control Centre : If the screen doesn't rotate on your iPhone, iPad, or iPod touch - Apple Support

It should be in the Control Center. I'll quickly update my iPad to double check.

Hey King Penguin how've you been? You're still too fast for me. You posted your reply as I was writting mine.

I'm ok, though other things have meant I haven't been posting as much on here

Glad to hear you're well. I've only just restarted posting myself.