It started with a cleanser. I was double cleansing like I'd been told to, with a nice balm in the first step, and my face just didn't get better. The flaky patches round my nose stayed and so did the redness.
Can you use an oil cleanser if you have seborrheic dermatitis?
It's worth avoiding. Most oil and balm cleansers are built on plant oils and fatty esters, and those are exactly what Malassezia, the yeast behind seb derm, feeds on. My balm was full of them. I was feeding the thing I was trying to get rid of, twice a night, because I was double cleansing (a very dedicated idiot, that was me).
What to use for a first cleanse instead
A micellar water does the same job without the oils. Bioderma Sensibio H2O is the one I'd start with, and the Garnier pink cap does the same thing for about half the price.
If you want just one face wash
Starting from scratch, Simple Kind to Skin Refreshing Facial Wash. It screens clean, it has no fragrance and it's about £3 in Boots and Superdrug. If your face is mid-flare and everything stings, the Etude SoonJung pH 6.5 Whip Cleanser, or Avène Tolérance if you can't face foam at all.
All of those are easy to get in the UK and all of them pass for Malassezia, with one small flag on the Avène that I explain in the full guide. There are 27 cleansers in there, with every ingredient list checked, in my best face wash for seborrheic dermatitis guide.