No-one replied to me do I gave it a go myself and it's pretty easy. I did need four hands though so I'd recommend getting a friend to help :-)
You will need to remove the oven door. Open the door fully, each of the two hinges has a hole within which you should place one of the metal pins provided with your cooker. If you don't have these pins, an Allen Key in each will do.
Now start to close the door, at about 45 degrees you should be able to pull the door out of the hinge housing.
Once removed, it's a case of slowly and methodically removing the relevant screws to replace the piece of glass required - it's pretty easy to do this but you must remember how to put the door back together. I took a few photos to ensure this was the case.
Once you have replaced the glass and reassembled the door, place the hinges back into the housing of the oven.
Open the door fully and now remove the pins (or Allen Keys) from the holes in the hinges.
The replacement is now complete.
Note: the operation to remove the oven door is also explained in the Smeg booklet which accompanies your oven.
Good Luck.
Chris, October 2006