We've confirmed it was not an accident.
Yesterday evening, after I changed the duvet cover and washed it, I emerged from the toilet to find a fresh patch of pee on the same spot, this time on the bed sheet. That same night, as we were settling down to sleep, Twinkle climbed on top of the blanket (new covers, just replaced) and peed on the same spot (directly over where my tummy is), right in front of our eyes. After a lot of shouting and scolding from me (which she ignored, as all cats do) and another cleaning job, I wake up this morning to find another pee spot, in the same location.
Needless to say, I am getting quite fed-up with this. If anyone has any idea, or has any experience with this sort of behaviour from cats, please offer me some advice. In the meantime, I'm going to lock Twinkle out of the bedroom, the only problem being that all 3 cats have to be locked out of the bedroom, which is quite a feat: at least one of them tends to slip past the gauntlet. There is, as people have commented, always the snip solution, but I'd rather find a concurrent solution to what I believe is a behavioural problem.