How on Earth?

All clipped together and easy when you know how. Got there in the end and all fixed!! I googled it, I YouTubed it all to no avail, which left me with only one of two options. One: pay someone else to replace the parts (not very likely) or two: cajole them apart with gentle levering and persuading. For a door mirror it's pretty complicated; including a puddle light, electric motors and indicator light too. The replacement genuine parts were supplied as agreed by the vendor, delivered to our door, but instead of a simple unclip and replace as he told me, it was quite a challenge.