I've just installed Meltano 1.13, loaded the carbon intensity data source into my postgre db but there is no transformation step like in the docs - what's the possible issue here?

This is intentional in the UI. We ship with default transformations and use those. Advanced users can add their own via the CLI (https://www.meltano.com/tutorials/create-custom-transforms-and-models.html#creating-custom-transforms-and-models). We plan to bring this option back to the UI when we have a first class widget/wizard to help users create them more easily. We've recently released a video about custom transforms, so here it is 😄

thanks for clarifying!