esc key

My researches suggest it's either:

key code 53

or

keystroke (ASCII character 27)

But it might of course be one or the other, or neither. Have a go!

Show us the script... Tell us what you're doing...

I would stick to the ascii key code. It seems to me that they work better most of the time. So the syntax to emulate the press is:

tell application "System Events"
tell process "the process you want to effect"
keystroke (ASCII character 27)
end tell
end tell

Also, you have to make sure that support for assistive devices is enabled. You can do this by going to System Preferences-->Universal Access-->Enable Access for Assistive Devices. Finally, if the command is part of a larger script, you may need to insert some delays after or before the keystroke to allow the application to react. Good Luck!

This is ever so slightly off-topic, but could you also twist this slightly so that the script waits, then when a certain keypress is detected it continues?

I don't think there is an AppleScript command to wait for a specific key down event and capture it. The usual way to delay a script if you want to do something else (like switch to another application to do something) is to use a display dialog. While the dialog is up, the script stops, when it is dismissed by clicking a button or pressing "Enter" for a default button, the script continues.