recently i picked up a bike that is too big for me , a 56 cm carerra vanquish for £250 rather than £550 list price that is mint. trouble is although i have put a saddle extender and flipped the stem to give me a reasonable cockpit i am getting lower back pain whilst riding.I am going to pop down to the LBS at the weekend to see if they can suggest anything but i am not sure what to do. Truth be told a 54 cm is comfortable and i am half a mind to sell the bike and cut my losses, i am pretty sure i can get my money back and look around for a better sized bike . Help please !!