Fortunately Nicky from The Flight Centre stepped in and resolved the issue of check-in for us, it seems the Latam are just plain incompetent and don't understand how flights are purchased nowadays. I would have provided feedback but inspite of asking for English language emails from Latam most of them still arrived in a.n.other language (Spanish / Portuguese, not sure to be honest) which I was unable to interpret. Much relief felt with flights finally sorted not many hours before needing to use them.
The drive to Heathrow and the parking was uneventful and unexciting, exactly what was required for that part of the journey.
Getting the boarding passes in the airport proved to be simple and successfully passed through baggage drop off and security.
The plane was packed and hot but seat back entertainment worked well.