It is correct that the glow plug light doubles as the engine management light. It could signify any number of things, but generally if its a fault on the injection system it will tend to come on and stay on, you'll have starting troubles or it'll be stuck in limp-home mode.
I suspect from your description of the problem that the throttle position sensor/throttle potentiometer or whatever you want to call it is on its way out. IIRC you have to replace the whole throttle pedal assembly as you cannot get the sensor separately, but its a straight swap and easy. The sensor sits at the top of the pedal with an electrical connection so there's no cable to mess with. It's quite a common fault on the Focus, and not too expensive to fix.
There will be a fault code logged for this, but I can't remember it. It would be worthwhile getting the codes read just to make sure its not something else - look up a Delphi diesel specialist in the Yellow Pages if you don't fancy taking it to a Ford dealer, they are usually a bit cheaper.