Advice on which CC to pay

Hi, I’m new to all this and this may sound like a silly question to many of you....I have a CC debt of just over £11k, which means I am paying £275 per month minimum payment, of which £170 is interest..and after looking at this site, I have taken out a new CC which offers me interest free credit on balance transfers for 28 months. I have transferred £2800 to the new interest free card (the max limit I was given) and set up a DD for the minimum payment. My question is this...which is my priority..paying off the existing high interest card, or the new card and transferring more of the old balance when I can?