It was definitely Portland Square. I can't find it on any of the old Ordinance survey maps but my Wishaw street directory for 1925 has it listed beside Cowies Square. This was (if you know the area) approximately near the top of the Beehive Brae, left hand side going up, so Portland Square must have been around there somewhere. Perhaps where Craigneuk Belhaven Church now stands.
Looking at the directory, there were quite a few shops there, so it may have just been a long row of buildings on the main road, rather than an actual "square". Almost all of the occupants were in the coal mining trade, so the houses may have been owned by the mining company. I will ask around, I have some friends who were born & raised around that area.
