2) While said GPS displays with DST/BST, it logs internally with ZULU/GMT time, which is what it downloads to GPX in, too.
3) Per 2), having carefully synched my camera's internal clock to the GPS display means that all my photo timestamps are an hour 'late'.
4) This makes geo-correlation inaccurate at best.
However, having figured that, I've got a reasonable chance of finishing the GPS/GPX -> flickr geotagging script soon...
ETA And I'm almost *certain* that PHP's default switched from Europe/London=GMT to Europe/London=BST somewhere in the middle of testing, and then *stuck* there.
There's something *really* odd in PHP's timezone support. The documentation's shite, for a start.