I've analysed all of the transactions stemming from that first transfer and produced a graph showing how the bitcoins got spread around and passed between different addresses. 
It's interesting how the transaction gets split up, then later quite a few pieces of the original transaction come together again. But the splits are probably the change that goes back to the same wallet, so it's not so surprising.And that suggests a way to determine (with a moderate degree of confidence) which part of a transaction is the "payment" and which is the "change".