Hey Gareth,
In NodeRed (as I am sure you are well aware) you can use either the inject node to post data to a Mendix app or a http request node to pull data from a Mendix app. In whihc ever direction you prefer in the Mendix application you can also publish a post API or consume a REST API. See the following docs for more information:
https://docs.mendix.com/howto/integration/publish-rest-service
https://docs.mendix.com/howto/integration/consume-a-rest-service