How to stop it bobbing around? It's really annoying to try to find a movie while it is swapping places.

Why not sort the Transfers list by a column like Name or Created or something? That way torrents will change position only when you add something that needs to be inserted somewhere in the list, and otherwise should stay static.

Sorry to be thick, but how would I do that?

That's actually one of the interface conventions Tixati does right LOL! Click on the column heading in the transfers list you want to sort by (you'll see the title highlights as a button). Click again to invert the sort. If you click on two different list headings, the sort appears to take account of both, so you can get many different subsort options.