I don't really know what you mean by "Article 50 does make it clear there would be no negotiation with the leaving member state.", Zacs. As you say, it says "In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State'"
I agree that the representative(s) of the leaving state would not be party to agreeing the consolidated position of the remaining countries, but surely once that position has been established the Treaty says that negotiations with the leaving state should occur. Of course what happened (near enough) was that the EU's position was laid out before the UK and that was about all the negotiating that took place (apart from which wine - French red or French white - they would have with dinner). But I believe A50 suggests that the process of concluding that agreement should involve negotiation.