Tried and failed. Two wees and two poos in the pants - Zachary seemingly unconcerned by these events and they certainly didn't prompt him to let us know next time he needed a wee. So decision taken to step back from potty training til after Christmas. Although intellectually he seems to understand the process, I don't think he yet really relates it to himself somehow and I'm guessing he still doesn't recognise the signs which tell him he's ready to do a wee or a poo. So we'll see how we get on in a couple of months.