I posted about this a while back, and got lots of suggestions.
The combinatuion of factors that cleared up my ds's rash were:
Bathed in chamomile tea while changing. Someone also suggested soaking a slik liner in it, if you have one.
Porridge oats, wrapped in a muslin in the bath. They make the water feel really soft and sor tof creamy - lovely!! It's very soothing.
I used Metanium too, and found it to be fantastic. i used it with a layer of sudocrem over the top to act as a barrier, as Metanium is not that thick, but it works better than anything else I tried.
Nappy off time if great too, if messy(!) esp. if the rash is bleeding or oozy nad creams won't stick to it. An hour of fresh air out in the garden, or indoors if you can face it - makes all the difference.
I did all these things at once, so couldn't say which one actually healed the nappy rash, (probably Metanium had the greatest effect)but as a combination it did the trick within 2-3 days.
I can never find Metanium in Boots, but have got it in several smaller chemists.
HTH