I am using 2.8.3-Wien to analyse two polygon layers, one with many more polygons (layer A) than the second. (layer B)

I need to know which of the polygons in A overlap with each of the polygons in B.

I've tried using Intersect, but I get a great big table with only half the data in it. It has all the polygons, labelled with data from B, but the fields which should say which polygon from A has overlapped is all null.

I had managed to get Intersect working with layers with less polygons, so is this just a processing power problem, or is there some other tool within QGIS that I can us?

I've not been using QGIS for long, so it's entirely possible I'm missing something really basic but vital.