I have noticed that a gap has appeared at the bottom of the passenger side wing mirror shell. I am not sure how this appeared. I recently had a cover over my Fiesta and when I pulled the cover of this may have dragged on the the shell moving it upwards. I  removed the mirror to get at the  clips  which secure the shell so that  I can  refit the shell. However, whenever  I raise a clip using a screwdriver it's very difficult to push the clip back once it's been raised, it just ends up returning to position. 

Any Ideas would be appreciated. THANKS.

Multiple screwdrivers is the best idea, so you can leave them wedge in under each clip to hold them all open simultaneously.

May be of some use

 

https://www.fordownersclub.com/forums/topic/67718-wing-mirror-indicator-bulb-replacement-mk7/

You have to engage the three lower tongues into the mirror housing first, then gently push the mirror cover upwards slightly to engage in the top clips, followed by the inner clip closest to the door glass, then press hard to fully fit in.

Removing the covers can be pretty frightening, I've never used screwdrivers or even removed the mirror glass, just followed the advice in the owners book.