How do you remove the timer control knob from a Zanussi FL1012?

I need to repalce the timer control knob on a Zanussi FL1012, I don't seem to be able to find any locking mechanisums that might hold it place. There is a very small detnet at rear of of main control knob with a small, whats look like a 'locking' device, but having held this in knob can still not be removed...very frustrating!