Lets say, for hypothesis' sake (and the fact it's true), that I work for ACN, am low down the ladder and after a couple years working here fancy trying to bust into BAH, Boston etc. Apart from relevant work experience on the job here, what other skills in terms of say Open University courses (like a maths degree) or acquiring a new language (mandarin for example) etc do you think would be considered the most desirable by said firms??????? - Thoughts with reasoning much appreiciated.

First, unless you have an outstanding academic career already forget it - going back to top up through OU simply won't cut it I'm afraid. Languages and so forth less important that evidence of leadership - this really is the important thing with the extras, so captain of a sports team, setting up an entrepreneurial business etc. Best advice would be to get 5 years at ACN then go do an MBA at a top BS like INSEAD or similar then apply - chances of making it otherwise very slim (but feasible). If you are good enough for MBB now, why are you in ACN is the obvious point.