Kelp (avaliable at Holland and Barrett) is supposed to be good for hair, but basically if you are healthy you will see an improvement in hair. I suggest you do Carol Vorderman's detox.
If you look in, say, Boots, there are special formulas. Don't buy these expensive ones, just buy own brand multivits as if you compare the ingredients the mutivits have the same as the expensive ones and often more !