Thanks for all your comments, own experiences and advice, every one.
She uses the money to buy crisps and sweets.
She gets "virtual" pocket money, ie her dad keeps a record of how much she has and she can draw on it but has to say what she wants to spend it on. This is because (a) we caught her stealing before and (b) she will binge on crisps and sweets if we let her. She is a bit overweight (not obese) and hates exercise.
She also gets real pocket money from her grandfather.
She does not have an ipod or a mobile phone.
We eat well in this family, all sitting round the table together every day eating food cooked from fresh (mostly). I do not buy biscuits or crisps unless for a special event, but I do make cakes/biscuits and occasionally buy chocolate that we all share (family of 5). Dd can make shortbread if she wants something sweet, and I usually serve a sweet dish as dessert so she is not sugar-starved. What I object to, and have told dd this, is her buying a load of junk and taking it to her bedroom to eat on her own! I sometimes find the evidence later, stuffed down the side of her bed or on the floor.
Maybe I ought to listen to Mozhe and not make such an issue of her stealing money from me. But every time it happens again, I think I should have dealt with it more severely last time.
To the person who said look after your handbag even if you take it with you to the loo: you are right, I know I know - it usually happens when I've been stupid enough to leave temptation on the dining room table for her - which is half the reason I'm so furious: angy with her, angry with me.
Dh has come home early from work today and has taken dd (protesting, natch) on a walk in order to be able to talk to her one-to-one. So we'll see what the outcome of that is.
I have tried to impress upon her how wrong it is to steal, and she must know or she wouldn't bother to lie about it.