Welcome to Flyertalk SimonX, welcome to the BA forum, it's good to see you here, though obviously disappointing to hear about your bags. Nevertheless I hope that we see plenty more of you in the future.

It's covered by IATA protocols, and basically the airline that flew you on the final sector that your bags were supposed to be on is responsible, even if it is 110% the fault of another airline. Codeshares are particularly nasty in respect of baggage, it can be very misleading to passengers in that area.

So going to SDQ then it will definitely be IB's responsibility. For the return, if the last sector was on BA metal then it goes to BA (and they deal with any IB internal issues). If it was IB back to London, then again it's IB's responsibility.

Their baggage centre has a UK number: 0203 684 3774. Doubtless it ends up somewhere in Spain however.