The answer is take a lot of photos

There are some advantages to being near the back of the cathedral with a pillar behind me. I wasn't going to disturb anyone by taking photos. I did take a couple with flash and one of them is on the left. The advantage is that the aperture is open for only a short space of time which means that blur from movement is much less obvious. The great disadvantage is that you light up the backs of heads near to you and this means that the exposure is affected and the background is much darker.

Just compare the exposure of the altar on the left with that on the right. The image is transformed, but if you want to use the ambient lighting then you have to use long exposures and this means you have to use a tripod. Even then if anyone moves they become a blur. Look at the two people on the right. They were obviously clapping their hands but the person in the middle also moved his head. What's the answer? Take a lot of photos and they may even stop clapping.

Happy snapping