bagpuss, just been through/going through something similar with dd (nearly 3) though hers isn't that red, nor that sore. She never wakes up at night, but occasionally itches herself or says its stingy in the bath - mostly at the front, and when I've checked she's had a bit of red or pinkness after her bath which is gone later on.
I've wormed her (Ovex), and used Canesten cream, and its hard to tell whether either have helped at all - both seemed to for a day or so, then its back to square one. I also checked her for worms at night with a torch for four nights in a row, and have checked every poo for a week (all in potty still): nothing. My final guess is eczema, so tonight on MN advice I put porridge in her bath - no itching since, so maybe thats it. She does have a flare up of eczema elsewhere ATM.
That said, I'm still changing towels daily, bed linen every couple of days, and hoovering more than usual - just in case. The whole idea of worms freaked me out totally (my one phobia, plus I'm pregnant and not able to take Ovex - though I have got some homeopathic stuff) so am not taking any chances.
All I can say is, you're definitely not alone - and if Ovex doesn't clear things up, I'd go to the GP. Hope she feels better soon!