This Study is in two parts, the first part is to make sure that combining a potential new treatment, cediranib (AZD2171), with a standard treatment (FOLFOX) for metastatic colorectal cancer is safe. Once this part is complete and it is decided that it is safe to continue the Study will the go on to look at the efficacy of the two drugs together. This will be done by studying two treatment options. One will be the standard treatment alone (FOLFOX) + dummy cediranib (AZD2171) tablets and the other will be the standard treatment (FOLFOX) + real cediranib (AZD2171) tablets. Using dummy tablets means the study is 'blinded' and that non-one can tell the difference between the two treatment groups. This kind of study design is done to try to avoid the chance that the results might be biased in some way. The overall aim of the second part of the study is to see if adding cediranib (AZD2171) to a standard treatment for Metastatic Colorectal Cancer (mCRC), in this case FOLFOX, gives better results. That is, it's better than giving standard treatment alone in helping to prevent progression of mCRC.