So I sent pictures of the Libre parts to my local waste collection people (same people who collect my sharps) to illustrate the different materials and electronic components. After a couple of discussions about biohazards and WEE, they came back to me telling me to put the sensor (and battery) in the sharps bin for incineration and the applicator in the normal domestic household waste.
Given the materials involved I was a bit surprised at the response, and I'm sure it's not 'best practice' but whatever!