From the ICAO 4444 manual (Air Traffic Control Procedures)
10.1.4 If an air traffic control clearance is not suitable to
the pilot-in-command of an aircraft, he may request and, if
practicable, obtain an amended clearance.
And from CAP413
1.13.5 The ATS messages listed below are to be read back in full by the pilot/driver. If a readback is not received the pilot/driver will be asked to do so. Similarly, the pilot/driver is expected to request that instructions are repeated or clarified if any are not fully understood.
Taxi/Towing Instructions
Level Instructions
Heading Instructions
Speed Instructions
Airways or Route Clearances
Approach Clearances
Runway-in-Use
Clearance to Enter, Land On, Take-Off On, Backtrack, Cross, or Hold Short of
any Active Runway
SSR Operating Instructions
Altimeter Settings
VDF Information
Frequency Changes
Type of Radar Service
Transition Levels
I liked the "driver" bit - thought it may apply to the Citroen Of The Skies, but then realised that vehicles have radios too. Darn.