Nan overruling parents

This is long so please bare with me. My mum raised myself and my sister by herself pretty much all our lives. We like to think she done a wonderful job as we have grown up appreciating everything we are given and all in all we are happy and healthy. She had rules just like every parent, we had to be home by dark, we had a takeaway once a fortnight on pay day, we had to sit and eat all our veggies or we weren't allowed any snacks etc. she worked hard to give us everything we needed and we had a great childhood with fun neighbours and tree climbing, imaginations and all that. Her and I clashed during my teen years but once I moved out, we became really close and can always have a laugh together. But since having my first child, mum has been overruling all my rules as a parent and does not listen to anything I say in regards to my child. Now I completely understand grandparents spoiling their grandkids. and I love how much she adores my daughter but it's never just one treat, or one present. I've asked her to tone it down and give her one thing instead of 100 things. Like one piece of chocolate or one toy. And she wouldn't be seen dead buying healthy food for dd. even though I've asked. From early on I said I didn't want dd to have chocolate, lollies or fruit drinks. Especially not soft drink. But from around 8 months old, mum has brought around chocolate for dd every visit. And not just one piece, it's always a few kinder surprise bars, a bag of mnms or a whatever else she has hidden. She deliberately hides chocolate in her bag and lets dd go through it (my pet hate is children going through people's bags!) and when I see that dd has found a piece, mum turns her head and pretends she doesn't notice she's found them. And it's always before tea time so dd never eats her tea. I've told her numerous times to only give her one bit, and sometimes none at all because she's been naughty or won't eat her tea. And mum won't bring anything the next visit but then after that seems to throw the rule back out the window. When dd was about a year old, everyone went out for tea (I was at work so I arrived once everyone had eaten) and I caught mum giving her a drink of her Diet Coke. When I told her I didn't want dd having any of it, mum said "but it's calmed her down". I then argued that there were other things she could have given her and mum cracked the shits with me, sulked and wouldn't talk to me. I've caught her also giving dd the last mouthful of her Diet Coke (she drinks around 2 600ml bottles a day) and again, I tell Her not to and she soon "forgets". Every time she takes her shopping, they come home with toys she only plays with for 5 mins before getting sick of them, a chocolate and a fruit drink. I no longer let her take her shopping as we only have a 2br house and the toy build up is out of hand even after numerous spring cleans. Whenever we are out, whether it's for lunch, shopping or at a family members house, mum makes a joke of it when I tell her not to do something, saying "I'm her nanny, I'm allowed". But if I get really angry, she again cracks it with ME and doesn't talk to me. If dd cries because she wants something, mum gives in instantly. There was a time she was letting dd climb on our couch over the edge (had she have fallen she could have broken a bone) and after telling them both to stop, dd flung her head back and got mum square in the nose. She actually went and got in her car and drove off without saying a word to me and didn't speak to me for a week. My daughter is a beautiful kid with lovely manners who we have worked hard to make that way despite the occasional terrible two tantrum but as soon as mums around, she goes feral and is a totally different kid. And now even my own family say how different and naughty she is when mums around and that mum brings out the worst in her. Which I hate even saying that but it's true. My mum has a terrible diet of snack bars, packets of chips, Diet Coke, cooked chips and McDonald's and everytime she asks to have dd for tea, it's maccas. Shes never ever cooked her anything healthy. She has not one veggie in her house, no bottles of water, no fruit and she makes gagging sounds when dd eats something healthy, and she does it infront of dd too. I don't care how shit her diet is, she can eat what she wants (although she's always so tired and drained from no nutrition) but I don't want her shitty habits rubbing off on my children. And if my nan had have done even a quarter of what my mum does with dd, mum would have resented my nan and stopped her having us kids. It's at the point now where even my very patient partner is sick of it and biting his toungue and I don't even want her to come over anymore because I hate what my child becomes when she's around. Ive told her she's not to take her to the supermarket because last time I went with them, she let dd run off and put a heap of crap in the trolley, I even watched her pretend she didn't see dd put a bag of lollies in there. And I was wondering why dd was running off on me and being really naughty every time we went out. I know why now! And one of the final straws was when I told mum dd had been naughty so she wasn't to have any chocolate, and the next time mum come over, I found a chocolate wrapper under the table. She had snuck dd chocolate while I was making a cuppa. I need to really get through to her, but every time I try to talk to her in a nice calm way, she ignores me, cracks the shits with me or argues that she can because she's "nanny". It's affecting our relationship and it makes me sad that she has no respect for us as parents because I know she would have HATED anyone doing this with us kids. Please help me find a way to get through to her without the arguments and the no speaking for a week and ruining our relationship!