So it all comes down to Cambridge were not correct in giving a "Clearance" while overflying the ATZ in Class G, glad that's sorted, only took 2 pages.
Simple - apart from the fact that the conclusion is incorrect. The following extracts from CAP 493: Manual of Air Traffic Services Part 1 indicate that controllers are permitted to issue clearances to aircraft ouside controlled airspace and whilst aircraft are not REQUIRED to obey there is an expectation that they will unless they advise that they cannot/will not.
Instructions issued by controllers to pilots operating outside controlled airspace are not mandatory; however, the services rely upon pilot compliance with the specified terms and conditions so as to promote a safer operating environment for all airspace users.
Outside Controlled Airspace
1.5 An Approach Control unit at an aerodrome outside controlled airspace shall provide ATS to aircraft, as determined by the Aerodrome Operator and approved by the CAA, from the time and place at which: arriving aircraft place themselves under the control of Approach Control until control is transferred to Aerodrome Control;
departing aircraft are taken over from Aerodrome Control until they no longer wish to receive a service or are 10 minutes flying time away from the aerodrome, whichever is the sooner;
overflying aircraft place themselves under the control of Approach Control until they are clear of the approach pattern and either no longer wish to receive a service or are 10 minutes flying time away from the aerodrome, whichever is the sooner.
Aircraft within an ATZ are required to comply with instructions from the ATC unit. Although flight in Class F and G airspace outside the ATZ is permitted without an ATC clearance, controllers will act on the basis that pilots will comply fully with their instructions in order to promote a safer operating environment for all airspace users.