Thanks for your input, all. I have found the answer in
AIC 35/1998 (Yellow 291).
3.5 Phraseology
Pilots should be aware that ATC use of the phrase 'No ATC Speed Restriction' does not absolve pilots from the responsibility to operate at speeds that would:
(d) comply with the airspace speed limit in Class E, F or G Airspace
So there we have it. Although it is there in black & white (or black & yellow, as the case may be), from Pom's reply and my own gut feeling, I still believe that crews
may be refering to the 'Airspace Speed Limit' when the question is asked.
Hippy