My mum, aunty (who's a nurse), godmother and such have all said the best treatment for nappy rash is fresh air.
Joseph has had it twice, so I was told to heat the room to body temp (36c) or warm enough for your bubs, lay them on a bed/changing mat/cloth etc and let them kick about freely with no nappy on, or maybe just a top but have nothing on below. This way, the skin gets to breathe and the rash will clear up a lot better.
However, I know the above is hard to do,when you have other things to do.
Worth a try though xx
p.s. Have you thought of changing the nappies? A few friends of mine had told me their babies were suffering nappy rash ad whatever they did, it stayed there. Then they changed to a diff brand of nappy and it cleared up. Dunno what they were using before but they are using pamper's dry now.