Finally got around to taking the dud floodlight down, took a short cut back through another of my gardens, and noticed that the floodlight there was identical. Switched it on, and guess what - that had failed as well, so, having the ladder and tools with me, I removed that one as well. Just a few hours burn time between them.
Turns out that they are 50W, not 20W as I thought, and made by Ledvance, which until recently was part of Osram, so not some unheard of cheap make.
Is there any reliable 50W LED out there?