This mirror unit is a like for like replacement for the original unit on the car. The replacement should take no more than 10 minutes (provided you have the correct tools).
First, remove the interior triangular plastic panel (behind 'tweeter' speaker) on the front door (you may break one or more of the three white plastic fixings - these are only 30p each so not to worry).
Next (very important) place a piece of tape over the gap above the interior panel (to prevent anything dropping into the door).
Now unplug the loom connection (lug on forward side needs to be lifted with screwdriver).
Next remove two of the hex/star headed bolts (size for socket is ST22). Now supporting what's left of the old mirror unit remove the third bolt positioned forward of the interior panel area. The old unit should lift off and unclip from top rear corner.
Carefully clean the area and fit the new unit in reverse order.