OK, so it turns out that my crappy sleeper had a crappy nappy - well, he did within five mins of being put on the mat to play (not when I was trying to get him to sleep) - so it looks like he was trying to tell me his tummy was feeling a bit ick and he needed a good poo! I just took him back to bed and he was asleep within minutes. Like I said ... just when you think you've got babies sussed out!