i know you have probably done this, but i now have very first hand experience of horribly sore bums and just this min have solved it.
ben is reacting to non-bio powder on his nappies - he was starting to get the worst nappy rash ever on his good skin - he still has a fungal nappy rash and prescribed cream for that. but doing nothing differently in our change routine or creams - as i can't interfere with the cream that is already there over night his rash - almost bleeding redness, sore in creases etc has gone. so the worse his bum got the more i used non-bio, and we started a vicous circle.
so now nappies washed with 1 tablespoon of sainsburys big box of green bio powder, 1 tablespoon of napisan, spinkle of lavender and capful of white vinegar,prewash, 60, extra rinse. hang up nappies to dry, fleece liners each get line of fairy washing up liquid on (vaseline and cream no good for fleece
) then put in bucket with kettle fulls of boiling water over them, stirred around with a chopstick every few hours, before going back in machine with other washing like towels, at 40 prewash and extra rinse with 1 tablespoon of bio powder.
like i said you have probably tried it, but i was thinking oif disps yesterday as it had to be cleared by this week as he is seeing a pead on monday and couldn't take him in cloth with a nappy rash. and it has worked like magic.