How do I add multiple versions (size / colour etc) of an item to one product listing?

You do not need to add multiple listings of the same product. Product attributes are additional characteristics of a product such as size and colour.

You can add attributes to your product by navigating to Dashboard > Products > Edit Products.
Scroll down to the ‘Attributes’ field and select the appropriate attribute type from a drop-down menu, and then add the specific details in the field on the left. Or you can create your own custom attribute.

For a guide on how to add attributes to your products – please see : How to add attributes