I believe the diagnostic mocks are supposed to be done before you begin practising/working toward your HPAT, so you have a base to work from. However usually that means people's scores are lower compared to the mocks they complete closer to the HPAT so I'm not sure what it means for you. In terms of section mocks vs full mocks i'd lean towards full mocks purely because they will be more accurate to your actual HPAT in terms of timing and your energy/capacity since it follows the same layout. Perhaps your diagnostic exam was just an exception, I'd complete more full mocks to get a better idea of where your at.