We are having a nighttime rash problem with Marcus

Every morning he is red and irritated looking on his inner thighs and around the groin area and sometimes on his bum cheeks too. It started a while ago and it would only be after overnight wear - he ended up with some nasty looking blisters on his testes and end of his doodle

WE cleared that up with no nappy during the day, but it took a while to do that as the nighttime nappy would cause them to flare up again.
I have to admit that we switched to disposables overnight because it wasn't going away

It did eventually clear up and once we finished with the disposables I put him back into cloth at night. The rashing came back straight away!

We've since had two separate stints of switching to disposables at night, and we've just gone back into cloth and it's happening again!

He literally begs me not to wipe him in the mornings when I change his nappy as the area looks really red and irritated so I'm sure it's hurting him
I've tried strip washing our night nappies - hot washing with no deteregent, washing with extra detergent, extra rinses, plenty of time in the sun - but it doesn't seem to have helped at all. I'd prefer not to use disposables, but I really don't know what to do about the problem.
I only use wool covers o/n too, so plenty breathable. Any suggestions?