The API is available at https://api.nsn.org.uk/v1/
Documentation homepage https://docs.api.nsn.org.uk/
The example below shows you how to authenticate against the API using CURL, and hit the /areas endpoint.
If you have an API key, enter it here to populate the examples:
curl \
--location \
--request GET 'https://api.nsn.org.uk/v1/areas' \
--header 'x-api-key: {your api key} '
Documentation homepage https://docs.api.nsn.org.uk/