OK, chaps. Pictures as asked. All these relate to the near-side wing mirror (passenger side on a RHD vehicle).
As it turns out all my woes came from getting a cheap mirror off Fleabay. Serves me right. Got a replacement mirror and plastic clip from Island 4x4 (no affiliation) the next day and it all just clipped together like clockwork.
Just so you know what to be wary of, here's a picture of the DUFF replacement mirror.
Notice the connectors are thin, rather than thick and also aren't bent downwards but go left to right.
This puts the connectors right into other working parts of the mirror unless you start bending things. Also, the mounting part of the back isn't raised enough so the clip doesn't fit easily. So tight in fact, you'd never get it on the car. Just bad tolerances and workmanship altogether.
Now for the GOOD stuff. Here is a picture of the real clip and mirror. Note the fatter connectors and the downward direction of the connectors to take them out of the way of the mirror working parts.
You actually put the clip on the circular mirror mechanism and then put the mirror on afterwards as described earlier in this thread but for illustration purposes I'll show how it fits on here. So, first right hand side slips under the loop of the clip and then the left hand side (outside) just slips under.
Here's the clip in place on the right hand side.
Here's the clip in place on the left hand side.
And here's what it looks like all done, although I've just noticed the antivibration clip visible on the top photos isn't on these.
Having shown that, take the 2 apart again. Here's the place where the plastic clip goes.
You just push the clip on and then sort of push around the rim until all the little lugs audibly click behind the lip.
After that, attach the heater wires and line up the antivibration tab with the slot and slide it in. Then clip in mirror into the plastic clip on the side nearest the car. A little inward pressure towards the car and slight push and it just clicks into place. Give it a tug and yank to make sure it is securely in place. Job done.
So, with the right parts it is a doddle. Just stay off Fleabay!