Error made and now my responsibility

Good day. In March I went for my annual blood tests. Referal was made by my GP and I had to have the tests done after nil per mouth. I went in to have the rest done then went home and had breakfast then went out. I received a call in the afternoon to say that my blood coagulated and I must please come in again. I said I would be in first thing in the morning as I had now eaten. In the meantime I contacted my friend, a medical technologist about the blood coagulation as I was concerned and she informed me that this would happen if the blood sample wasn't shaken enough. I went back the next morning and they took extra blood using a syringe. A month later when I realised the medical aid wasn't going to pay for the extra tests I contacted them and was told there was no mistake made by Lancet - they had apparently not taken enough blood - so naturally this wasn't cleared up. Well now they are pressurising me for payment. Because of their error be it blood coagulating because of non-shaking or not enough blood taken, the medical aid won t pay as there was now no authorisation for the second batch of tests done the next day. I would like to know where to go from here and what they are going to do about Lancet's error. Thank you