Shops product has stained my ugg boot but they are giving less than they cost!!

[Jadex91]

Can someone please give advice? Would appreciate it so much. I Have ugg boots tjay cost £160, I was out shopping and picked up some oven cleaner. It leaked in the basket and has stained one of my boots. It won't come out and they are completely ruined. I told the store manager and he said I will get the price back if I write to head office, I included photos and they said due to the sales of goods act they are only offering me £60 as they are nearly three years old. If I didn't pick this up it wouldnt of happened and they are in excellent condition. They sell now for £190. What do I do? Please help x