I just popped in to Decathlon and bought two of the proper tubes. What I will do now though, is maybe carry the 700x32/47 tube as a spare since it's smaller and lighter and I'd normally just patch on the trail/road anyway so it'd need to be a big failure before I put another tube in.
Well...
I have a 27.5 x 1.75 inner tube on a 27.5 x 2.6 tyres, I just can't be arsed to change it.