I had a similar experience whilst under training - I was on a solo flight. I was on finals, having made the correct radio calls, when I heard a motor glider call joining late finals. I looked out to base, and saw him heading straight for me in my 9 o'clock. What did I do? Frankly, I panicked, called that I was orbiting, and turned into him so that I could keep him in sight. He called back that what I was doing wasn't very clever. He was right. Thankfully, we both landed safely. My instructor hadn't seen/heard the incident, but told me that I should have gone around. I went off to the gliding club to aoplogise, to be met halfway by the motor glider pilot who was coming to see me to apologise! I guess we were both in the wrong; he shouldn't have tried to cut in front, and I should have gone round. I learned about flying from that..