More to the point
It should have been made clear when you were appointed whether the management point was a permanent addition to your salary, and pensionable, or just a temporary increase for a specified period. You should be notified in writing of any change to your salary. With the new arrangements for salaries that are to come into force in 2006, all schools will be reviewing staffing arrangements and the present "M" points are to be phased out.
After a transition period, some teachers will see their pensionable salary fall. Some who are nearing retirement may choose to leave before that happens.
I am more concerned about your reference to "managing" a nursery. There is a fine line between management and leadership and you might want to find out whether you're being asked to undertake a leadership role in excess of what can be required of someone only paid for management responsibilities.
The review of school staffing would be an ideal time to clear this up. It might be worth talking to the local secretary of your professional association. Also look at job adverts to see how similar posts are advertised by other schools.