With Mavericks I would unlock an encrypted disk using "Disk Utility", but in El Capitan the "Unlock" button isn't there.
No, just moved them.
Disk Utility > Menu > File > Unlock
(Embarrassingly I found this item after posting the question and using command line option to unlock the drive)
Command line, open "Terminal"
hdiutil attach /dev/diskXsY
Where X,Y are system specific numbers like 1,2,3.
You can get the button back by open the "Disk Utility" and then right click on the Toolbar in the Disk Utility window. Choose "Customize Toolbar". Now just drag and drop the "Unlock" button to a position where you want it. Click "Done" and you are ready to go again. Just mark the encrypted drive, click "Unlock" and you can either enter in your password or use a password from the keychain.