Thanks to all of you that have posted replies - using some of your ideas and finding my old multimeter I have now solved the problem. Alas i hav discarded the unit, but i did note that it did indeed have day/night modes via a switch on the side.
Once again, my sincere thanks to all of you for replying, your input did help in a real way
Once again, my sincere thanks to all of you for replying, your input did help in a real way