Tbh, poolofjoy I think you are past the point of 'gently, gently'. You have an 11 year old who is (was?) 55.3kg, which is at 99th centile for BMI and is classified as obese.
I'm not saying that to be unkind, really. I'm saying it because as soon as somebody says anything about treats you could buy to soften the blow, you jump on it, and you've ordered it. You're concerned about limiting her if she's not had a big dinner at school one day, etc. You have to realise that she got this way because you over fed her. She will not starve if she is hungry, I promise you! Grouchy, yes, grumpy, certainly. Starving, no.
My DDs got into the habit of saying they were starving whenever they were bored/tired/at a loose end/in between activities I started to say "no, you aren't starving, you ate at , at the very most you are 'peckish'. Starving is having no food and having to eat beans and rice for every meal, even when you do have food, then having to walk 5 miles for water." Then they'd say "I'm peckish" 
DD2 hates eating healthier food, but it's good for her and we remind her that by doing that we are saving her years of dieting in the future. We don't talk about dieting now, we talk about "Move more, eat sensible portions."