"They need winter clothes from September onwards, what does she think they wear for three months"
Wel if it's from Matalan, perhaps best it is given at christmas... 3m ought to be the most it lasts... 
Seriously, some items Matalan, Tesco, Asda, Primark etc are fine.... but some - especially for girls is O M G! Ugly and just bleugh!
We ALL know that.
mamatomany, sounds like this issue is a symptom and not a cause. You have bigger issues in your relationship with your Mum than clothes.
Use the leggings under things, use it as craft clothes and the really hideous tops, if you can take them back to matalan and exchange them for anything of any remote use, fine, if not, then charity shop for them.
You are somehow relying on a woman that can only be relied upon to upset you and let you down.
If you know this, then plan around it and isolate the problem. Stop looking to your mother to live up to your hopes, she is not going to start doing that.
The annoyance you feel is due to your unrealistic expectations of her. Accept this and learn from it and it'll get easier.