Thought I should report back:

Outbound - LHR (BA) GRU (Avianca) BSB. Flight was 2 1/2 + hours late leaving LHR, which made things a bit stressful as we had allowed 3hr 10 mins transfer time and we were on different tickets, as the LHR flight was a redemption. BA were going to telex GRU (so they said) to arrange for someone to meet us (we were in first) but no-one was waiting for us. Longish lines at immigration, but I managed to persuade a rather reluctant airport employee to let us go straight to the front. Fortunately Avianca's check in desks are straight above where you exit on arrival and no-one seemed bothered that we arrived after check in should have closed - in fact there were several people behind us in the queue going to BSB. So all OK in the end, albeit after being more stressed than is ideal at the start of a holiday! Had the BA flight not been so late, 3hrs 10 mins would have been way more than necessary.

"Return" - BSB (Tam) GRU (TK) EZE - on a US Airways redemption with 2 hrs 20 mins to connect. Bags were checked all the way through, no need to pick them up and recheck them in GRU. TAM issued us with boarding passes for both segments but we were told to go to the *A desk at GRU, which we did. They referred us to the TK desk, where our TAM boarding passes were swapped for TK ones. We were then directed to immigration and then the GOL lounge, where we had probably an hour after wandering around to see what was what at GRU (answer, nothing much exciting). TK in business was pretty good, in my limited experience of business class travel and the bags arrived fine, albeit nearly last on the carousel (by which time we were getting a bit worried they had gone astray). Otherwise all very easy.