3 Month workmanship warantee is a LIE!!!

Firstly I don't receive my monthly statement- I paid my bill this based on an estimate amount. Secondly i sent my phone in for repairs they replace the entire screen and front piece but when i get it back its still faulty so I send it back - Then i get told its my battery so i purchase a brand new battery and the phone works for two weeks and then the screen goes black so I send the phone in again as the 3 months workmanship warantee on the screen is still valid and now i get an sms saying i must pay..There is no way im paying for there mess up- This has to be sorted under the warantee as they touched the screen even when that was not an issue -- I will go to the ombudsman if i have to ...