Ok... knew I just neeeded more coffee: Solution with @media

added to css:

@media (max-width: 817px) {
  .header {
    width: 100%;
    min-width:auto;
  }
  .main {
    width: 100%;
    min-width:auto;
  }
  .mainCol { min-width: auto; }
}

this resets things so unwanted min widths etc go away, I also added:

@media (max-width: 649px) {
  #sideNav { display: none;}
}

to remove the side bar if screen is very small...