Thanks for all that. We will probably be landing in T3 based on seeing when our DFW flight has been landing, so we will probably just have day packs and check everything else through, bit good to know that we can probably leave central London 5:30 to 6ish to make it back. Should be enough time to see whatever is showing at the V&A or Tate Modern and stop at a Pierre Herme before heading back.