To answer some of the questions:
We have some LLS silk liners, which help clear the some of the redness (especially the skin above his willy), but as they aren't stay-dry, he gets sore in his creases instead.
We do use Bio powder, but I'm wondering if that is part of the problem. I might try some non-Bio if the strip wash etc doesn't work.
I do use disposable wipes, but have done since birth and he hasn't had much of an issue with them. I get the best ones I can (which are ironically the tesco own ones) and have tested them one my skin when it's sore and they don't sting.
The rash is red, like something has burned his bum
There are a few spots of soreness, but I think this is due to wetness.
I'm going to to try some minkee liners, but I think I will have to ditch synthetics.
So what natural fabrics will keep him dry as he needs stay-dry too? Is it just wool interlock?