I've decided to start a new thread derived from my previous one about starting reloading, in which I controversially mentioned my intention to go off-piste and try and develop a load for these French DL1 bullets that I'd heard very good things about. Since then, I've done a little more research into what these things are and where they come from. In France, they're exclusively used by a company called Freyr Munitions, but I discovered that DL stands for "Degol", or Mr Wim Degol, who manufactures custom bullets outside Liege/Leuwen. It turns out that these are also available from a company called Johannsen in Germany, and they're not so cagey about the details. So here they are for anyone who may be interested:
Given that Mssrs Freyr weren't particularly helpful, and then turned weirdly secretive about everything, I've contacted Degol directly, so we'll see what they say.