Hi Simon and welcome. There is no way for a user to get the collet off center. It fits in a cone and the cone is supposed to center it as the nut pushes it down into the cone. If it is spinning off center then it was machined that way. By the way, if it is off center it should be vibrating like crazy. You can tell if it is off center from the main axis by clamping something to the base and leaving a paper thin gap between whatever that is and the bit. Then as you slowly turn the bit you should be able to see the gap change. Likewise if it isn't 90* to the base. The gap will be wider at the top or bottom. If either of those is true then the router is junk. Return it and get a refund. The saying that you get what you pay for is pretty much universally true with only shades of gray to it.