Turned the key this morning. It sounded like the starter motor was spinning but the engine wasn't turning over. Any ideas?

 

Steve,

Your XJ6 is known as an X300. Very different in all ways to the cars in here.

A quick repost in the X300 section will get you answers related to your car.

My best guess, is the reduction drive has failed. From memory??, they are a nylon gear arrangement.

Others with better knowledge will answer in the X300 area once the time zones align better.