did you use an auth.json file in the project root? I am not sure if Heroku will pick up on that but it should. That file allows you to give credentials for a composer repo. Something like:
@Mick79 As a note: it’s probably better to add your authentication credentials as an environment variable rather than hard-coding your email address and password in your application’s composer.json file.
I recently wrote a blog post on how to do this (albeit for Bitbucket private repositories), but the concept is the same—you’d just create an environment variable in your Heroku application called COMPOSER_AUTH with the same JSON blob @ejdelmonico provided.
@MICK79 - having this information on a public even private repo is not actually nice,
i solved it buy providing an environmental variable to Heroku COMPOSER_AUTH=<auth.json content>
but if you are getting invalid JSON, go the dashboard and remove the quote surrounding the JSON