Lee- you are a good mummy caring about your darling's bottom. Bad mummies wouldn't worry...
I have been using Sudocrem and it actually hasn't been working (when it has in the past). I also use olive oil to create that barrier so wee can't get to it. Is the rash a red spotty one near the top of her genitals? Only asking as my DD has the same and it won't go away either.