When I attempt to start the car, all I get is a "whirring" sound. Engine does not even try to turn over. I've never had a similar problem. My first guess is that the Bendix is not engaging.

Any thoughts on the symptoms and/or my guess?

From what I can see, it looks like the starter is a bear to replace.

 

photog, may i recommend a link on the FAQ thread, very top of the XK8/R forum, there's a link titled 'Car won't start? - read this first"

sounds like cylinder wash... go there and read, good luck!