It's taken me a couple of evenings, but I've now managed to get it working, at least at a basic level. I used Adam Franco's script (here) to automagically generate a KML file from a geotagged Flickr set, then I used OpenLayers to combine that KML file with OSM. The whole shebang is here
Next I would like to generalise it a bit more, so I could do the same with different sets, and it would be better if I knew how to embed a slippy map in a blog post, instead of grabbing an image like I have here.
But as a first attempt I'm quite pleased with it. I know there are lots of people out there who can do this stuff in their sleep, with both hands tied behind their back. But just for now, I'm feeling a bit chuffed that I'm still able to pull this kind of thing together.
There's a slightly enhanced version which can pick up "any" KML file here using my Sustrans Coast & Castles trip as an example.