Right then, let's try this again... So as you can see from the photo below, the LED units are much thicker than the OEM ones, and will be very obvious in daylight :
Also, that big plastic 'stalk' with the retaining spring in it makes it nigh on impossible for them to fit in my bumper, and they wouldn't be coming out again if I'd forced them in. I figured I'd try to make a hybrid, using the LED element from one attached to the 'base' of the OEM one (cheers Mark, for giving me spares

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My craft skills are renowned on here, so I have to say I wasn't confident... This is how the LED units split :
They don't break down any more than that, but then you need the heat sink for the 18 LEDs, which is the copper coloured band.
The OEM unit is one piece of plastic with a clear plastic window. Using a junior hacksaw with a fine tooth blade I set to work giving me a frame to mount the LED unit to. The design of the OEM one is such that, with some careful cutting, it is possible to keep the retaining clip, which is handy... Here is the split unit :
I then had to mount the LED housing. The OEM units are a tight fit in the bumper opening though, so I had to trim a couple of millimetres off each of the long sides on the 'base' of the LED housing. Here's the two bits together, not in place though. The LEDs will be set back from the window in the OEM base and behind their own window in turn, but they are so bright I figured that wouldn't matter.
I then used my trusty silicone adhesive to stick the two pieces together - it's strong, fills gaps, and it's waterproof. The latter being important as there is no gasket on the OEM units and the watertight seal is normally created by the bulb holder. Silicone is not very elegant though, and it doesn't look too pretty. Here's a couple of pics of the finished article :
Fitted to the bulb holder on the car, working

I used black masking tape to wrap the connector to the LED housing, and to keep the other end in the original bulb holder. I then used a small cable tie to attach the wire you can see to the bulb holder to stop things moving around in the bumper. No pics of that though sorry...
Finally both fitted, looking nice and flush. Pretty bright, even during the day :
That shot is taken pretty low down, standing upright you can't see them.
And here's a pic at night - insanely bright 8)
In total it probably took me about 90mins, including quite a bit of head scratching time. I've also completed my LED switchover by using the bulbs that were in the holders I removed for the boot and the glovebox. Job done.