Moving the rear view mirror
The mirror on the RHD 451, like the RHD 450, was too low for some people. For me
the mirror sits right in the centre of the windscreen creating a major blindspot. On the
450 you could buy a 3rd party shorter stalk that made the mirror sit higher.
The 451 rear view mirror is attached to the windscreen so we need to adapt the fitment.
Shortening The Mirror Stem
Thanks to Paul Duncan for the pictures and idea for this mod
Detach the mirror and stalk from the windscreen by pushing gently upwards at base of stalk.
Remove mirror by gently twisting loose from ball on mirror arm

Mark the mirror arm on stalk just above the base for ball connector.
Cut it with a hacksaw and use a mitre box if you have one.

Take care to preserve the slight angle on the ball of the arm.

Clean up the newly cut edges with a file and sandpaper to form a flat base.

Mark the other end of the stalk.

If you look at the back of the stalk you will see the flat edge you need to cut just above.

Clean up the newly cut edges with a file and sandpaper to form a flat base.

Don't get confused and do anything to the side shown below, this attaches to the windscreen.

Using 2 part epoxy glue or contact adhesive, glue the 2 parts together as shown below.
Don't use superglue, it's quick but it doesn't have any flex to it (low modulus of elasticity).
The every day vibration through the mirror will soon cause a super glue joint to fail.

Because good adhesives take time to cure, it is a good idea to hold the 2 pieces in place
as they joint sets. You don't want to hold it together for a hours, tape is a good alternative.

When it has fully cured you should have a very strong joint. Push the mirror onto the ball
and slide the newly created shortened stem over the bracket on the windscreen.

Ensure the stem is pushed all the way down onto the bracket. Set your mirror to suit. Job done.
Additional Info
To ensure it was secure I decided to make a 2mm hole through the ball, all the way through the base plate. I then inserted a 10mm number 4 screw through the base plate and into the ball section.

I did this as I found the glue I used was still allowing a tiny amount of lateral movement.
As the rear view mirror is adjusted regularly in my car I decided it needed reinforcement.
Using The 450 Short Mirror Stem
Take the standard 450 mirror shortening kit from Tuning Base, ditch the screw, you
won't need it. What you will need is a 60mm M5, bolt, penny washer and a nyloc nut.

Use a 5mm drill bit to punch a hole through the front roof trim.
To remove the roof trim have a look at this guide here.

Assemble the mount and push the bolt in place.

Attach the washer and the nyloc nut and tighten.

Refit the roof trim.

Job done although it is worth noting that this setup can vibrate as you drive.
Using An Alternative Mirror
Most cars these days have the rear view mirror bonded to the windscreen so it would be a very simple task to get a different mirror and swap it over. Measure the stalk of the new mirror to ensure it would sit in the right place before bonding it in place.
It is important that you do not use superglue to bond the new rear view mirror. Most car shops including Halfords will sell you rear view mirror bonding adhesive.
When searching for your replacement mirror, remember that not all mirrors are the same.
Most these days have a toggle that dips the mirror which is handy but some high end cars have auto dimming mirrors which would just require a switched power feed to it to make it work.
You can also get rear view mirrors with hidden screens for reversing cameras or LCDs designed for rear parking sensors. The final development is a mirror with a built in courtesy light, nice.
Confirmed Fitments
Smart Roadster (evilution)
Citroen C4 8153.SG (Stanis1292)
Mk1 Audi TT 8N7 857 511 01C (Margaronis)
Peugeot 206 2005 onward with no sensors (Sean M)
Stanis1292 said
"don't limit yourself to Citroen - Magna Donnelly supply mirrors to BMW, Chrysler,
Daimler, Ferrari, Fiat, Ford, General Motors, Honda, Hyundai, Mazda, Mitsubishi, Porsche,
Peugeot, Renault-Nissan, Suzuki, Tata, Toyota & Volkswagen as well as many others"
If you try and succeed with any other mirrors, let me know and I'll add it to the list.