Are you sure it's not thrush? Impossible to say without saying, it's just that every time we've had a bad nappy rash it's actually turned out to be thrush - like if it's not responding to the usual treatments (air time, barrier cream, etc) then it's actually thrush. It's usually localised in a few splodges and it just gets worse and worse until you realise what's going on and get some canestan. Good luck!