This post has the answer - Modifying headers with IIS7 Application Request Routing
Need to enable preserveHostHeader - can't see how you do that in the UI but this works
Run this from command line to update Machine/webroot/apphost config
%windir%\system32\inetsrv\appcmd.exe set config -section:system.webServer/proxy -preserveHostHeader:true /commit:apphost