Well, after picking up my car from the stealers - where it had its new Lambda sensor fitted this morning - I spent the rest of my lunch break fitting these newfangled LED sidelights. It wasn't as physically tricky accessing the bulb holders as I remembered, just a bit fiddly. Somehow I managed to drop one of the two existing 'blue glass' sidelights into the void though... whoops (so if I need to swap out my LEDs for whatever reason, my free gift will actually come in useful!).
The thing I didn't expect though was that the LEDs don't necessary light even when they're in the right way... I found I had to turn the car on and turn the headlights on before they would light the first time. Once they were working and mounted though, they both lit consistently when I turned the knob one stop - I didn't have to turn the key. Perhaps the odd behaviour was due to the canbus thingey I keep reading about - it seemed to 'reset' when I turned the car on? Anyway, they're in now and working well. It was bright sunlight for once outside, so I couldn't judge the brightness but they look much whiter than the blue bulbs that they replaced.
I'll keep you posted on whether I get any bulb warnings or ought.