I think it's a bit harsh to cut your mum out of your life over money issues.
She's had a problem in the past and this woman giving her the money, coupled with you not asking for it all the time, was obviously too much temptation for her. It has caused you some hardship, but is it worth ending a mother/daughter relationship over?
Besides the money issue, is she a loving mother? A good grandma? Did she provide for you as a child? Is she good to talk to? Is she supportive?
I would suggest to her that she asks her GP for counsellor to help her overcome her issues with money and surrounding her crap upbringing. Unless she deals with her shitty childhood she may be unable to move on.
As for the money itself - I would work out a direct debit of whatever she can afford. It may only be a token amount, but you are not doing it for that but the principle of the matter.
It's nearly Christmas. I suggest you have a serious chat with her about just how this has affected you and your Christmas budget - after all people who can afford Christmas don't have to start up a Christmas club do they? So it's not like you can just write that money off.
However £400 is not worth destroying your family for. You are all your mother has left by the sounds of it. You also need to take responsibility, you knew what your mother was like around money and yet you made that decision to trust her. You might think that you should be able to by now, but having all that background information, that decision was still yours. I daresay you won't be making the same mistake again.
But she is your mother, not a friend. And I think that on this occasion family should pull together and offer both of you the support you need.