Blank LCD (*64/*56 models pre 2006)
Battery for LCD display - SR44(w)/V357 (Duracell code is 357/303 you will see this on the Duracell packaging). Remove display by it's edges with your fingers or blunt knife. Do not pull out too far as wires are attached. Remove black backing disk with a 5p piece and replace battery. A paper clip will help you pop out the old battery. Replace display, having carefully replaced backing disk. Do not lose the backing disk, we cannot replace it.
Do not buy LR44. SR44 battery also known as GPS76E. If your unit is pre-2003 and you replace the battery and the display still does not function, you may need a D357. This is a nearly identical battery only slightly thicker.
Available from most high street electrical shops and many on-line retailers.