[Icecast] How do I combine my ssl certs?

[Robert Chalmers]

I have https/ssl on my site ok, but it uses two certificates from letsencrypt which renew automatically every three months.
However - Icecase says.

ssl-certificate
If specified, this points to the location of a file that contains both the X.509 private and public key. This is required for HTTPS support to be enabled. Please note that the user Icecast is running as must be able to read the file. Failing to ensure this will cause a “Invalid cert file” WARN message, just as if the file wasn’t there.

So what is meant here. How do I combine my keys into a file to satisfy this?

Thanks
Robert