Thank you both for you help. I've done as suggested and got to the next step 
The field is showing up as invalid in the Slice Machine UI, although I understand that is expected based on this post.
I've pushed both a slice and a custom type after adding directly in the respective model.json files, however after pushing the updates to the repo the field does not display in the Prismic UI for the repo:
I tried deleting and recreating both the custom type and the page containing the slice to which I had added the catalog field, but it didn't seem to help.
Do I need to be on a specific version of the Slice Machine in order for this to work?
Any further help would be much appreciated as I'm not sure how to proceed.
Thanks,
Paul