Updating data certificate tags as data download regenerates

[peterkwells]

The website content for the data download page includes tags for issued date. We are now publishing weekly so this tag needs updating somehow.

Previous content, now removed:

  <p>
      The downloadable data was generated from the live platform on
      <span property="dct:issued" content='2014-12-10' datatype='xsd:date'><b>10 December 2014</b></a>.
  </p>

@Floppy aware and pondering how to do