Can I just hijack the sore bum bit?
We had a major problem with DS2 when he got blistered, open sores as a newborn. It was awful and we went through every treatment (sudocrem, then metanium, then thrush treatment, then steroid cream etc etc.)
Then a helpful HV came along with a brilliant suggestion: essentially it had turned into a bedsore and needed treating as such and because poo/wee was constantly abrading the skin ever couple of hours, it would never heal. so she suggested something called Cavilon, which is used for treating bedsores and burns I think...
It's basically a plastic spray (it is designed for skin) that coats the sore with a breathable layer. It gradually washes away but you need to clean and respray each change... You can get it on prescription but you will have to ask you GP or HV for it - I don't think you can buy it over the counter.
This wiped out all the competition and cleared up the blisters/sores very quickly - as I recall they improved within hours and got better within 24-48 hours.
Hope you can get hold of some