aha - I think I have (partially) found the problem! The directory structure I was using consisted of a directory containing full-res images, and also one containing thumbs (legacy from an old album). This thumbs directory was being loaded as a sub-album, and seems to have been causing the google maps extension problems. Removing this folder brought the markers back to the map.
However, I am still only seeing on the map those photos on the current page of the album, rather than the album as a whole. For example:
http://bbarker.co.uk/zenphoto/Offas_Dyke_Cycle/
All images are coded, but different markers appear for each album page. I was looking the the db tables - I see the the exif data for each image read into a table. That would (perhaps does) presumably allow for really advanced searching - so far I have only found the search box at the top of the window, but I'm still exploring. I was trying to see whether it would be possible to run a query on the db for all coded images in all images in the gallery and show them on a map, or all coded images before a certain date etc (select lat,lon from thistable where lat is not null and date > somedate...etc)
Very cool whatever the answer,
Ben