You can wean yourself off this OP. But don't try and do it all in one go. Try it in stages, otherwise the craving for the chocolate will be worse if you try to go cold turkey.
Firstly, don't buy the big slabs. I am not in UK so I am not sure what a giant treat size is, but I will assume its a slab. So next time, buy that chocolate you like, but in a smaller size, but buy one. And then look for some chocolate bars that have cereal in or similar. I wont say brand names here as you may not get them there, but the muesli bar type of thing with chocolate on top. If its the sugar you are craving, then this sort of thing might just work. I know it sounds silly, but while you are eating it, think of it that you are eating is the chocolate you really prefer.
I had a very big problem when I was a bit younger with drinking lemon water. I know it is all the rage again now, but this did quite a lot of damage to me. For years it was the only thing that I would drink, and it resulted in me losing all the enamel off my teeth, as well quite serious stomach issues, peptic ulcer and now severe ibs. I had to stop drinking it, but silly as it seems as its really a healthy thing, I was horribly addicted. I even got help for it as I would be in a foul mood if I couldn't have any etc. So what I was told was to just drink plain water, then think (but you have to focus) that what you are drinking is the lemon water. I wont say it was an overnight cure, it took a little time, first I tried lukewarm water, then cold water, just a few different ways, and after a while it really did work. There is so much lemon water options available now, every now and then I am tempted to grab one off the shelf, but then I put it back every time. I know it isn't the same, but I can relate as you feel like you have lost all your willpower to this thing that can fit into your hand. So just cutting it all out just in one go, I don't think will be the easiest option. So little by little, just be aware of how its makes you feel AFTER you have eaten it (as you most certainly do not get that same feeling BEFORE you do).
Good luck OP. Please tell your dh/partner about this, so he can help you with this. If someone else is buying the items, they are enabling in way without even knowing.