This is an interesting update from the Chartered Institute of Taxation (or CIOT) relaying the contents of its recent discussions with HMRC's representatives.  

There has been a drive in recent years to encourage taxpayers who may have undeclared sources of income to voluntarily come forward and regularise their tax position via various tax disclosure facilities. These avenues of disclosure have steadily grown fewer and more punitive in recent years and this update shows this trend to be continuing.  

HMRC state in their recent update to CIOT that should any taxpayers have utilised such disclosure facilities but still have not declared all of their income that they will be contacted shortly to have the opportunity to advance any any mitigating factors before HMRC consider the issue of imposing financial penalties on those taxpayers (and chasing the tax due) for such oversights.  

As ever one cannot feel that full and early disclosure in such matters is by far the best course of action.