Got it!!! After major stripping down of the board, cleaning, testing boost power supply and checking chips plus replacing capacitors....i.e. a whole saga......put in decent new chip sockets for every chip, not easy as I'm not sure they make some of the sizes anymore, and built the board back up monitoring the supply load. Found a suspect chip, replaced and it is now all back up and running perfectly. Unlike stereo displays or clock displays these units don't tend to get heavy use so the display is bright and legible which is a very happy result! I guess the answer is perseverance and being bloody minded.