You could give them a go if you take a multivitamin too. I find that they definitely taste off if the vitamins degrade but I have eaten some 2-3 months after but probably wouldn't go much longer. You can usually smell that they are not right so you can just chuck them without having to make up and taste. The official line was don't have them but it's a best before so it won't do you harm it just might not taste nice Food labelling terms - Live Well - NHS Choices
I'm in the same boat except my shakes are dated 02/2014. Wasn't sure if having them 4 months past their date would be dangerous or not. I can't remember how they smell when they are good but I guess if they "STINK" then it would be best to bin then. Going to check mine now and if they smell and taste ok I'll be able to save myself from spending on 2 weeks worth