I am trying to add the raspbian repo to my sources.list file.
after adding the repo and running
$ apt-get update
I get the message
E: The repository 'Index of /raspbian/ wheezy Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Don't forget Debian wheezy is very old.. Have you checked the required dependencies are available & won't clash with your unstated release of Ubuntu-MATE (the link you provided refers to more than one release).
As Wheezy is so old, I can't look up its requirements (why I used the oldest jessie-8 currently available not wheezy-7), and you intend loading a non-Debian packaged version, so you'll need to do your own evaluation as to risks & breakage it may cause, but I'd not recommend it.
I'd for sure be trying to use a Ubuntu packaged version
If you need a specific package from Raspberry Pi OS, you can use Synaptic to download the .deb files, and transfer them over to MATE and install the packages with Gdebi, or...
Thanks for your suggestion!
I did try this but the Raspbian .Deb is for armfh not arm64 so it failed.
I did a bit more reading and someone on GitHub ran into the same problem.
They had to compile cec utils form source with additional steps.
This is our of my league but will get around to learning when I've put my Arduino down!
well this shows that gpg key is not available or correctly installed you could tell it explicitly to trust the repo but since this is extremly not recommended and highly insecure unless you specificly can trust the repo aka you own it.