I love the smell of a leaked legal service opinion in the morning

The conclusion: the EU Council legal service says that it's OK for the Brexit deal to be concluded with the UK by the EU only, not needing Member States' participation

The reasoning: the legal service distinguishes between competences reserved to Member States, and competences shared with the EU. For the latter, the EU has the choice of concluding an EU-only treaty.

The legal service summarises the prior case law backing up its interpretation.

It refers to the Weddell case, concerning competence to conclude a treaty on the Antarctic environment, where Member States *could* be involved. But that was based on the particular legal features of the Antarctic treaty; and the UK is not Antarctica.