Why is"Nous NE souvenons de notre dernier concert"? Surely "nous NOUS souvenons de... is correct?

You are missing the second "nous" (the reflexive pronoun) as well as "pas" for the negation:

Nous ne nous souvenons pas de notre dernier concert.

Or, in the positive form (as you have correctly surmised):

Nous nous souvenons de notre dernier concert.

I hope that helps, -- Chris (not a native speaker).

The question was "Nous nous rappelons de notre dernier concert. = Nous ________ notre dernier concert. (We remember our last concert.)" This, didn't seem to really explain what I needed to do, but I answered "Nous nous souvenons de notre dernier concert". YOUR answer said "no", it should be "Nous NE souvenons de notre dernier concert"! Can you now see why I was so confused?!

Please excuse my madness! Yes, I can see I did make a typo....humble pie! So sorry!