How to download a file in web forms

Hi Folks,
I am trying to download a file in a VS2012 ASP.Net 5 web forms application.   The user clicks a button, the file gets generated and stored in a local directory and I then pick it up and try to initiate a download using Response.TransmitFile.
The button is inside an update panel as I need to refresh an area of the form afterwards.  If I move the button outside of the update panel it works.
The problem is that this uses Response.End() which throws an exception to terminate. Following this exception the web app throws a javascript error   Unhandled exception at line 1  http://localhost..../bundles///...    JavaScript runtime error: unable to get property PRM_Timouterror of null reference.
Does anyone know what I need to do in order to get this to work inside the updatepanel?
Regards
Andy