Liam,

Per the AFCAD manual, if you move around the airport in slew mode, you can pick out the locations where you want the parking spaces by hitting "o" on the keyboard and a parking spot will show up on the AFCAD file.  You must have the AFCAD file for EGLL open to do this.  Also recommend making a backup of your original file of EGLL in case you want to revert back to it.

If your question is regarding visual scenery displayed in fs2002, working in AFCAD will not result in any changes in the visual scenery - to make such changes you'll need to use a scenery design program, such as FS Scenery Creator.  This way you can put down concrete aprons wherever you desire.

Erik