After more random guesswork it turns out we had added

     <httpProtocol>

      <customHeaders>       

         <add name="X-UA-Compatible" value="IE=EmulateIE7"/>     

      </customHeaders>    

   </httpProtocol>

to our web.config to fix some other compatibility issues.

Adding

 <meta http-equiv="X-UA-Compatible" content="IE=11">

to the page has fixed it, but not sure why we have to do this?