Standard practice for trail/enduro riding. A front 21" tube will fit in a rear and inflate enough to hold it on the bead and get you home.
I was obviously not using the right search key words. Mmmm, not as practical on a soft adventure bike as I first thought. Thanks for finding what you did. Andy
Need a first aid kit too. If I'm camping, I use one of these: https://www.selkbaguk.com/original-4g/4936-selkbag-original-4g-colour-lemon-chrome.html
Same, already starting to plan with my boss that I take some short-notice leave and just f*ck off with a tent, a bag and a phone.