Rangemaster User Manuals here...
http://www.rangemaster.co.uk/index_Rangemaster_Manuals.htm
On my cooker clock (5 buttons) I depress together nos 2 & 3 from the left and use + and - to adjust the time. Yours may be similar or perhaps; different... I do this after switching off and on again to reset any stored delay times to zero - the display flashes on and off.
Peccavi
December 2008