I've been awkwardly saying "Aitch Five Oh En" in my head, but my work colleague says "Aitch Son", which even though it's a 5 and not an S sort of makes sense given the relationship to JSON :)

It certainly is - but with a much narrower intended scope of use. It's really not intended as a data transport mechanism for example. It's also homologous with JSON, XML is more flexible in its data representation. And finally, it makes use of HTML5 conventions, such as prefixing custom tags with a hyphen delimited string, data-* attributes etc.