It's all quite clear (in the UK): unless the original restriction is restated as part of a second instruction/clearance/restriction, it no longer applies. The second instruction/clearance/restriction can be on the same or a new frequency. A common sense qualifier to this I can think of is something like "Remain clear of controlled airspace, call 123.45". Clearly a restriction like that still applies until cleared otherwise.
[This message has been edited by Hew Jampton (edited 30 January 2001).]