Try to use white zinc cream called "sudocream". You can buy it in Rossman and Aplteka also.
It is in light gray ,roun receptacle, whit inscription on the red background. Try this, it is effective for napkin rash.
When our little one had napkin rash we decide to change nappy brand from Pampers to Rossmans babydream nappy, and it hepled.
when i was in England i tried all the supermarket brands like tesco and sainsburys but they were awful, I'm using pampers premium here which are rarther expencive but never had a problem with them before, I may try some others when i next go to get some from rossman