the exact year in this case is one of the unanswerables, but I think it might well be late 1920s, possibly early 1930s. I don't know who the girls are either, but I have an idea as to why they are sitting in the middle of a field peeling potatoes, and that is that this is one of the camping trip/boating holiday photos of which there are quite a number in this particular lot.
But what might be more of mystery is the message written on the reverse of the sister photo to the one shown (same shot but the girls are looking at the camera).
One possible explanation is that the note was written some years later in war time, the curtains were black out curtains and the photo in question was already considered suitable for writing reminders on. Acocks Green is in the Birmingham area (in case you're wondering).