Well i've heard it'll cost around £60 if I got to cycleworld (or something named like that) and they'll sort my bike out to suit me perfectly, taking into account, height weight ect...
£60 seems very cheap. Just check what they are doing for that. If you wanted to go triple, then I would have though that involved a new bottom bracket or at least a new axle, new front lever and a new triple chainset, plus labour. That will come to a lot more than £60.