Myprotein complaint: faulty item which I want to return and money back

Ordered a pea protein and when i opened and used it the packet had an odd smell, the powder was very strange and texture and when i mixed it there was a layer of froth and a brown ring which didn’t look right as I have ordered protein before. I contacted the site and returned one package as i had two, paid £6.95 and asked if i could get the postage back as I didn’t have a choice really and didn’t want the other bag to be a problem. I was told to wait and someone will make contact about the bag i opened and when told them the issues they said go to your GP and get a sick note and only then they will refund me the bag cost and the postage even though i told them i haven’t taken much of so can’t go to my GP for anything. I’m sick of having to argue when all i’m asking for is my money and take all the protein off me instead i’m getting silly people ask for sill things as mentioned.