How is the strip on the outside of the doors held on as our passenger front has come loose a bit, next to where the door closes

Which trim are you on about, the body coloured one on the bottom edge of the doors or the black trim 1/3rd the way up?

If it's the black trim then they are stuck on with double sided foam tape, which will probably mean a new trim as they often bend when you remove them - it is also a messy time consuming job to do.

If it is just the chrome bit then super glue it back on - I asked the cost of replacement and was told around £80. I then used super glue as the edge was lifting - job done.

If it is the body coloured trim along the bottom of the door, one of mine was replaced under warranty. It seems they are held on with some screws that locate into plastic clips that can break. The plastic clips weren't available as spare parts, so an entire new trim had to be installed. It was a very expensive fix for what was really a very minor, simple problem.

Yes its the strip that's about a foot up the doors like a rubbing strip colour coded on ours