Ok thanks for your help. Have finally found a helpful person at BA that has assured me that i would be treated as though in transit and that my luggage should be checked straight through onto the NRT-MEX flight.

I was hoping someone would be able to answer my question above regarding whether open dates or dummy dates are best for my santiago-easter island return flights. I currently have dummy dates due to the fact i used the booking tool but would i be best changing this so that they are open-dated. What are the advantages/disadvantages of each??