QGIS » QGIS Application

I'm using a new install of QGIS 2.18.2 on an iMac running OSX 10.10.5.

I create a new project. I use the OpenLayers plugin to show an OSM background of a part of Mayfair in London. I now use the Georeferencer to add an 9Mb .png (copy of a web map) and import three points. All the SRS/CRS are set to EPSG:3857. Transformation settings are linear.

• the 9Mb map I'm trying to transform
• the points I have previously saved
• a screen copy of the transformation settings
• a screen copy of the QGIS window showing the GCP table and the two maps
  At this point I run the transform and I get the message: Transform failed: Failed to compute GCP transform: Transform is not solvable.

I have no idea of what is going wrong here. I have managed to use exactly the same process (with a different but very similar .png) and it has worked in the past (but not recently), but any attempt to add a second file usually seems to fail. Trying to add the map that did work to this project also failed in the same way. Many other images/points have failed as well. I've also reduced the PNG to a 1.6Mb jpg and this also fails in the same way. I have removed all plugins other than the OpenLayers plugin and it still fails.