Richard,
The endpoint to use is: https://api.companieshouse.gov.uk/company/XXXXXXXX.
Does the try it out functionality return any results? The host specified in the try it out functionality should not have the undefined on the end, this is a bug in the site, so that will not work.
When setting the Authorization request property did you append a : to the end of the api key?
The new Companies House REST API uses standard Basic HTTP Authorisation which consists of a username:password pair. The API key is the username but we are not setting a password, but the delimiter between the two must be present hence adding the : to the end of the api key. The key including the : must be base 64 encoded.
Let me know if this helps.
Thanks
Mark.