Somehow PI's Calculate method was no longer running during the normal workbook recalculate.
In Settings I had to change Automatic Update command to Full Calculate and then back again.
My solution was to find an old version of the same excel spreadsheet which was not having thses issuesbefore I archived it, which is of similar size and complexity to the current version.
I opened this spreadsheet, checked that this was still working and autoupdating with current installation of MS Excel and any new automatic office updates (which it did work with), and then just reopened the original spreadsheet. In my instance I was using a particular add-in called PI Datalink.
I had a case of this just now on Excel 2010: a particular spreadsheet that would not auto-recalculate.