I'm 65 in Feb 13, I've just decided not to retire.
Do i have to notify my employer in writing?
If so is there a legal minimum period of warning I must give them ?
Or.... are they legally obliged to approach me on the subject ?
Since the change in the law and retirement reform, you need to do nothing. Just keep working and they will not and should not approach you on the subject. (unless you happen to be an exempt profession such as police officer)