Update: what's working for me is Coconut oil - get one of the organic ones, Tesco sells one I think. It's like a white butter that melts in the hand. The effects last for a few hours. As soon as it starts itching again put more on. Half an hour after putting some on you'll get itching as it starts to work, then it dies away, then when it stops working the itching starts again. I keep a small jar beside the bed so that I can put a bit on if I wake up. One jar lasts for ages. I also ended up with a little bit of the same rash on my back where I sweat a lot when exercising.
Winter - two duvets and underwear, it's easy for the genitals to get very hot and the itching starts again pretty quickly. My solution was to ditch the underwear just to keep the temperature down.
I also tried vinegar - peeled off a layer of skin after going all scabby, no long-term benefits. Lemon juice - again, a scabby layer of skin that peeled off but actually made it worse, my theory being that the lemon juice contained sugars that the fungus thrived on. Milk of Magnesia - did nothing. Garlic - very painful, skin peeled off, no long-term benefits. Tea-tree oil - same as garlic, skin peeled, no benefits but there are dangerous possible side effects including hormonal dysfunction and poor coordination. Over the counter Daktarin containing miconazole (any ingredient ending in 'Azole' is likely to combat the two main types of fungus causing the problem) - very very slowly cleared it up but I wasn't keen on the chemicals in it getting through the thin skin of my scrotum and it was getting pretty expensive.