I totally feel your pain (and understand that feeling of rage)! I have a nighttime yowler myself, Aztec, but his is due to a combo of cognitive disfunction, deafness, and diabetes.
I know you say Twiggy has been to the vet, but has her T4 level been checked recently? Increased yowling can sometimes be a symptom of hyperthyroidism.
What about increasing Twiggy's playtime just before bed? A good session of chasing a furry or feathery wand toy, or laser pointer if she prefers that, followed by a bedtime snack, might help her sleep through the night better.
Feline behaviourist Pam Johnson-Bennett talks about cats and meowing on her website: http://www.catbehaviorassociates.com...aning-of-meow/