<p>How can I get a ASP.NET (inc MVC) application talking to a Flex UI over AMF.  I am wanting to push approx 100+ records around at a time and AMF would appear to be the way forward, but there doesn't appear to be anything obvious.</p>

If you're pressed for time, you can just use the RemoteObject to hit a compiled DLL  (like WebORB - its free for .NET, but you need a VS copy above Express to compile your classes that you want to expose to Flex)
and Retrieve the object that way...
Obviously your objects should have a DAL in place or be generated so you can communicate with your database.
But i suggest using Cairngorm for any data intensive Flex application. It isn't simple and development won't feel as fast, but once you understand it, things go alot smoother and it just feels right. I could go into the details, but there are people that are much smarter than I am that have already explained it, in depth. Someone like yourself should be able to grasp the concepts pretty quickly.
here are the links to learning WebORB and Cairngorm:
weborb :
http://www.themidnightcoders.com/weborb/
cairngorm :
http://opensource.adobe.com/wiki/display/cairngorm/Cairngorm
learning Cairngorm :
http://www.adobe.com/devnet/flex/articles/cairngorm_pt1.html