There are various regulations with regard to this. They can be summarised as follows:
firstly, it must be properly supported all the way along its entire length;
flexible ducting must be demonstrably more supported than rigid ducting;
ducting must run straight without any distortion or sagging;
ducting must not bend in any way at the point in which it connects with ceiling grilles and outlet terminals;
ducting must be secured to the roof outlet terminal.