I've created some travel time isochrones using the 'OSM Tools' plugin in QGIS. I now need to remove overlapping sections of the isochrone layer so that I'm left with discrete polygons for each isochrone (in this case travel time to a central point). This is so I can calculate the proportion of a 'district' that is within x and y and z, mins of the city centre. So for example here are my isochrones
If I now remove the 'shortest travel time' (yellow polygon) it's clear that all other polygons depicting travel times in excess of the yellow polygon are beneath:
I would like to cookie cut out the longer travel time polygons so that none of the individual isochrones overlap i.e. my layer looks like this (with the yellow isochrone hidden):
I had thought that the difference or symmetrical differnece tool would do the trick but no joy. Is there an easy way to cut/clip polygons on the same layer from within QGIS?