You can get an API key by visiting https://code.google.com/apis/console and clicking "API Access". You will then need to switch on the custom search API on the "Services" tab.

To access the API requires an OAuth Access Token - what I really needed however was a refresh token as they don't expire and can be used to get an access token every time the script ran.

You can use Googles OAuth Playground to generate a token.

1. Create a Google App Console - https://console.cloud.google.com/apis/dashboard > create an app > Enable the APIs you want > go to credentials > Create Credentials > OAuth Client Id > Other > Copy your credentials
2. Edit the credential and add the authorized redirect URI https://developers.google.com/oauthplayground
3. Go to https://developers.google.com/oauthplayground
4. Click the settings icon > check "Use your own OAuth credentials" and set your OAuth Client credentials from step 1.
5. Select the scopes you want
6. Exchange authorization code for tokens - You now can get a refresh token that won't expire!

If you don't use the "Use your own OAuth credentials", your refresh token will expire after 24 hours.