Ta I will do it on Thursday night then :-)

I think the SZR office in the DNATA building next to the Metropolitan (not the Emirates building) is only open in the day time whereas the airport offices are 24/7 (but may be subject to the normal arbitrary unannounced closures). Google e-gate Dubai and you can find the times.

The e-gate office is a little tricky to find as it's not well signed. It's on the first floor - turn left at the top of the escalators and follow the railing all the way around as far as you can go (past the office for religious travel). It is the little office at the very front and centre of the building overlooking SZR. Always fast and friendly in my experience.