Ah, finally, found the film I was after on YouTube: https://www.youtube.com/watch?v=y0zieGfrFYw
It is in Polish, a comedy, from 1960, about a couple who get married and have a plot of land on which they wish to build a house - except that there is a huge mound of earth on the plot....
Well, it is about a buried tank.
Fast forward to about 25 minutes (and also see a rare view of a Communist era beer kiosk).
Trevor