My unit is around 23 yrs old, never replaced the battery. Previous power cuts fine - just had to reset day/time and on/off settings. Could always use select on CH to go from Off/Auto/Once/Cont (in auto mode will follow timed on/off); HW the same. Following recent power trip (only took out sockets including heating but didn't affect lights) and then a power cut (of a few seconds) unit now only allows either HW or HW+CH. Select moves both sides through list. Is there (a) a reset method- I assume the operation resulting from power cut wont restore jst be removing power and removing/replacing battery?, or (b) is there a direct replacement without needing rewiring?