You should not be experiencing any fumes, assuming the co detector is servicable other possibilities is either the engine has just been painted on maintenance or there is a small oil weep dripping onto the exhaust and that is being smelt as it burns off in the cabin....
If the CO detecter is passed it sell by date, check that exhaust is not being sucked in through the cabin side vent, also check the nose wheel steeering boots to see if they are split...... they could also be what caulleystone is getting
Did you try any experiments inflight, such as cabin heat off, airvent closed etc and did it improve?
Ask them if they have an engineer to get him to whip the cowls off and have a look... better safe than dead