Yes it's true! We have not broken any promises - we promised a referendum on the EU constitution. We still want a referendum on the full constitution of the EU as constituted from the various treaties since the Treaty of Rome - ie. an “in or out” referendum - which is a much, much wider matter for referendum than the very narrow amending treaty - the EU Reform treaty.
There is a principled reason to ask for a referendum on the EU as a whole as it is constituted and to reject a spurious referendum purporting to be one on EU membership but in actual fact being on a series of amendments.
Many thanks indeed to MatGB on Liberal Democrat Voice for pointing out that, after flailing away writing hundreds of words on this subject, I may possibly, just conceivably, have stumbled upon a reasonably persuasive few sentences at long last.