Thanks sooo much for all the replies about Clement's rash. Been busy today, so only now trying to answer questions/update!
His rash didn't get any worse overnight, and that was with a cloth nappy over the disposable, plus a wrap (as he wees OUT of a disposable every night!), which further adds to the dislike of paper nappies, since they'll not cope with him at nights! So can't be heat making the infection return, since he had 2 nappies on and it didn't look worse!
But by later today it has spread a bit further, rather red scrotum - and oddly enough, he's ended up with a patch on his thigh (as before), but this time the creases of his legs are free of rash. So it's spread to thigh, with clear area between!! And it was specifically the fact that he had it in the creases of his legs before that made the GP even more convinced it was fungal/thrush (since it was in moist/warm place). Sooo, I'm thinking this makes it all the more likely to be a contact rash. And maybe the patch on this thigh is from where the Cotton Tots was on his thigh (over the disposable) overnight.
I had wondered at once stage if he'd become allergic to bamboo (or bamboozles, in particular) since that is what he wore 95% of the time, so that's why I boil-washed the Cotton Tots, so he could try something different. And most of the CT nappies hadn't been worn when he had the thrush, so it's very unlikely that anything was re-infecting him (even in spite of the 95 degree wash), since they'd been put away and not used for weeks/months previously.
But, I'm wondering if it's possible that the seam where the booster is attached to the nappy could be rubbing on his skin? It's an overlocked edge there, isn't it? And a bit rough. And even when folded over, still a bulk of cloth. And I know a lot of eczema suffers (or parents with E children) talk about seams bothering them, and turning clothing inside out so the seams don't make them itch. So just maybe he's very sensitive to that, as the wavy line of redness is right where that booster attaches to the nappy. I think he's probably been wearing a size 2 Tots Bots (either Bamboo or cotton) each time the rash has come back, so maybe a seam free smooth nappy (something with a loose booster, or velour lined) wouldn't be so bad. But whatever the case, I think if I can get the rash to go away, I'll keep him in disposables for a few weeks to see if it well and truly stays away with them, before trying any form of cloth again.
As for fleece liners, he's allergic to them anyway, so it was a paper liner he was wearing. Not tried minki liners on him, but when I used minki wipe on him he went a bit red, so I think he might react to it too!
Sorry to bug everyone with my long detailed messages, but I usually find I get more helpful/intelligent advice from other mums, rather than GPs, who can often be very clueless!