Plonky before you start messing about with food elimination try to reduce all contact with soaps, washing powders, and synthetic fibres etc as those are the triggers here.
Use non-bio washing powder and double rinse clothes and towels
Wash and double rinse all new clothes before wearing
Cotton clothes only, avoid polyester
Clean the bath out with a cloth and hot water before use, every time
No baby bath bubble/oils, soaps, body washes, just warm water and a flannel. The bath sponge was also an irritant so we ditched that.
Aveeno cream all over after bathing, also first thing in morning, last thing at night, and at nappy changes. Oilatum is a waste of time. It made no difference at all here.
Reduce baths to once every 3 days at most. If I bathed dd every day her skin would bleed.
Give it a few days, and be really diligent about the cream. Hopefully you'll see an improvement.