In reply to Bookworm's question:
Slots will not normally be revised within 30 mins of departure unless a flight has been declared "ready for improvement" (RFI) or if ATC tell us that the flight is ready to accept any short notice improvement in which case they send us an REA message. This normally contains a minimum line up time.
The RDY message died earlier this year and is no longer used - ATC were often unable to cope with short notice changes to the slot, hence the new system.
If you do not have an Ops department and your flight is in SIP wanted status any slot improvement proposal will be sent to the AIS at the departure airfield or to the originator of the flight plan.
Of course all of this goes out of the window for emergency "stop the traffic" type situations.
I hope this helps clear things up for you.