online clothing store fraud

[Anabel01]

Hi all,
I am asking for advise as I have no idea what should I do

I have purchased 4 piece lingerie set from UK online clothing store (not one of big brands), it was terrible and too small so I have emailed company 3 times asking where I can return it. I have finally managed to open return claim online and posted items in perfect condition. 3 days after I have received it back, in damaged box and with missing 2 pieces and letter saying that my return has been rejected because the set is incomplete and they will only accept it when I send them missing pieces. I have tried calling them and sent them a lot of emails but did not succeed. So they are basically thieves, not only they have rejected my return but they have sent it back to me as incomplete set

In my whole life I have returned 100's of clothes because of fit (It is difficult for me to find something that will fit) and I have never had an issue
I have posted 3 other clothing items for 3 different stores and all have been accepted

I don't know what to do :(