what are the overide buttons for how do they work?

I have a brown potterton ep200 slider central heating and boiler control unit about 15 to 20 years old, I have just moved in and would like to know what the 2 square overide buttons are for and how they work , I dont have a user manual , Any ideas on how they work would be greatfully welcome, if any one knows where I might also get a user manual I would be greatful , Thanks for trying to help.Re-submitted due to error should read ep2000.

Keith Patterson
December 2006