Command + R not working to factory reset MacBook Air

This is what you should be aiming for:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

TO successfully enter Recovery mode, boot the Mac and, when you hear the boot chime, press and hold one of the key combinations:

❝ On an Intel-based Mac:

• If you used Command-R to start up from the local Recovery system, you get the current version of the most recently installed macOS.
• If you used Option-Command-R to start up from Internet Recovery, you might get the latest macOS that is compatible with your Mac. 
• If you used Shift-Option-Command-R to start up from Internet Recovery, you might get the macOS that came with your Mac, or the closest version still available. ❞

Exactly what Mac model and year are you trying to reset?

What is/was the macOS version running on it?

• Is your Mac an Intel or Apple Silicon processor?
• Are you first shutting down the Mac, instead of a Restart?

Starting up in Recovery for each processor is here:

How to start up from macOS Recovery - Apple Support

And the Support Article to reset to Factory Settings is here:

Erase your Mac and reset it to factory settings - Apple Support

Follow up -

If this Mac is a desktop model - iMac, mini or Pro tower - and you are using a wireless keyboard, then switch to a wired USB keyboard for better luck when trying to reboot in recovery. Very often a wireless keyboard may not work for the process.

GM58 wrote:

As I said in the question it's a MacBook Air, so your suggestion doesn't apply, I'm afraid. Thanks for answering though.

Apologies. I haven't had my US RDA of caffeine this morning. ☕️

And you're welcome.