12 month Unconditional Discharge, in Aug 2007, do I have to disclose this on my job app form in Feb 2011?

Are you sure you mean this? If you had a conditional discharge (for twelve months), then it must be mentioned if you are required to disclose all convictions or if the prospective employers need an enhanced CRB. If you were discharged unconditionally, it means that you were found not guilty, in which case there is no reason to mention it. However, if you are unconditionally discharged, no period of time is mentioned, so I ume it was not the case.