I'm only guessing, but it sounds fungal. Get some LugolsIodine and Clarkia and start on both. Can't hurt and might help. If it is fungal, you'll notice an improvement with the neem soap and oregano oil. That's what finally cleared up ringworm on my daughter's shoulder blade. We tried everything, but nothing worked until we used the neem and oregano oil. You might be re-infecting yourself, which is why I suggested washing everything in hot water with bleach.
Funny it doesn't itch when you're sweaty. I would think you would be.
Hope what I've suggested helps!!
PS. I totally feel for you! I've had more than a few yeast infections so I can totally appreciate the torturous agony of burning and itching "down there".
Humaworm sells anti-candida herbs as well as anti-parasitic herbs. I dunno if you'll have a problem getting in UK or not, but you could try cross-posting on the humaworm site.
But in the meantime, give the neem soap (it also comes in an oil you can try topically, but I have to warn you, it smells vile!) and oregano oil a shot. It worked quickly on my daughter and cleared the rash up in about a week after suffering through it for a couple of months using otc creams and gentian violet. I'm a believer in the power of neem and oregano oil for fungal stuff.