Honeywell CM927 control unit- LCD display issues LCD

The LCD display on the remote unit displays all symbols. I am unable to change/ alter the date, time or program. We haven't used the unit in a couple of months as had the heating turned off. While not in use the batteries were removed. I've tried a few sets of new batteries to no effect. The heating comes on, I'm assuming using the timings previously set. The heating also comes on when I switch it to manual. No matter what mode I select or buttons I press all symbols are displayed on the LCD screen. Please can anyone help as I can't replace the unit. I did try gently heating the unit on the radiator as I read somewhere it can get damp inside but still not fixed the problem. Thank you