Is the Isle of Man not included in the source json?

well spotted - yes it's missing. Data from https://bost.ocks.org/mike/map/uk.json

Oct 2022 Update The problem I was describing here is no longer visible. I assume there has been an update to D3's path or curve functions which solves it. Olivia

Oct 2022 Update (as above) The problem I was describing here is no longer visible. I assume there has been an update to D3's path or curve functions which solves it. Olivia