Found the answer myself - you just go through the stargate and then come back again.

Opponent is still in arena, but door now open.

Hi Just starting up the game again after a verrrry long break! I've just been killed in the Battle Arena Maximus. On restarting I can see my opponent flying around in the ring, but I can'g get back in - the door is closed as if the combat was still going on inside. Can anyone tell me how I get back ...