Multiple Entry Custom Values : Find modified ID?
Don't know if this will help, but at least you can try it. :)

The custom() hook is called after the db write on a custom table, so when the field is writen, you can't know which was the change, so you can't have the id (or at least I think so).

Instead of custom() you can try the pre() hook. Check if the Enjoyment Level has changed (use api to read the field in the db and compare with the $param), and if it had, update the Updated field.

Check this links that posibly can help you:

Setting and getting custom field values from within hooks

Hook pre