What I found was that the
vb footer code contains a section on:
<!-- auto DST correction code -->
and it was this code that was being executed. However, it was trying to post to a script in the wrong location which was producing a 404 error and redirecting to the main index page.
Appears to be some code which is executed once for a time change (which we just had) and it only kicked in after daylight savings took effect.