For the past decade, I’ve been resisting the urge to buy into the new skincare ‘must have’, or ‘Zeitgeist’ addition to my already extensive, expensive and time-consuming skincare routine. Serum – now we are supposed to buy Serum. On top of everything else. We are all already cleansing, toning, moisturising, eye-treating, scrubbing, masquing and Argo oiling it up, right? Surely that’s enough; surely that will keep horrendous complexions and the dreaded ageing at bay? Nope. Its not enough, ladies. Now you need to add a (often very expensive) serum into our daily mix. Applied after cleansing, and just before moisturising, it is designed to ‘boost’…