Obviously the problem could be with WP Geo but I thought I should register the issue here:
WP Geo shows a map in the post edit screen so you can easily click and drop in a pointer or type in lat lon manually.
I set up a tab in Tabify and moved the WP Geo metabox to it.
In the post edit screen the map appears but only shows one Google map tile at a time to the left of the map area - the rest is just a grey background.
I can drag the map a around so new tiles appear but only one at a time and in the same part of the map area.
Disabling tabs on my posts returns everything to normal.