Sounds like the common camshaft position actuator or the filter element blocked restriction oil flow channel to vvt camshaft hub/sprocket.sometimes the small filter in the actuator can block causing that same issue..also lack of routine oil and filter changes using cheap oil filters can too also...there was also a field remedy for the same issue as detailed below...might help you out..
Insignia 2009...A16XER| A18XER
Complaint: Knocking noise after cold start. This noise appears sporadically after cold start and the engine switched off for a long period of time
Cause: The camshaft position actuator activation starts too early before the necessary oil pressure is reached
In case of customer complaint program engine ECU with modified software. Software will be available as of 11th of May 2009, TIS2Web 5.5.
FunctionalGroup:J - Engine
Complaint Group:14 - Noises or Vibrations
Well givin the problem you describe a service wouldnt solve it unless the oil condition in that engine was bad to cause that.. if they have serviced it then that would rule out oil / filter ..if they used quality fluids and filter...but as above it could be an easy job for them to remove the actuator and check the filter element or clean it up and pop it back in and then see if the problem is resolved....besides you should have 30day warranty as standard when buying a car from a car dealer..so they should in theory be investigating this issue and hopefully resolve it..