Aha! I've re-discovered it - in the House of Commons Library Briefing Paper No. 07212.

To save others spending time searching, here's the applicable piece in Section 5 regarding the type of referendum into which the 2016 EU Referendum falls....

"This Bill requires a referendum to be held on the question of the UK’s continued membership of the European Union (EU) before the end of 2017. It does not contain any requirement for the UK Government to implement the results of the referendum, nor set a time limit by which a vote to leave the EU should be implemented. Instead, this is a type of referendum known as pre-legislative or consultative, which enables the electorate to voice an opinion which then influences the Government in its policy decisions. The referendums held in Scotland, Wales and Northern Ireland in 1997 and 1998 are examples of this type, where opinion was tested before legislation was introduced. The UK does not have constitutional provisions which would require the results of a referendum to be implemented, unlike, for example, the Republic of Ireland, where the circumstances in which a binding referendum should be held are set out in its constitution."

For anyone - in particular, in-denial Brexshitters - who might be sufficiently interested in what the precise status of the referendum was, rather than rely on their own creatively-over-fertile imaginations, it can be found here.