I just had a thought. To be fair, that bang is not straight away after opening a hot water tap. You open the tap, water starts flowing and after maybe 3-4 seconds you can hear that bang. So it doesn't look like it is typical water hammer. My feeling is that this bang happens when boiler shuts central heating flow to prioritise hot water. Also that pressure jump on central heating when that happening... You don't think it might be something to do with boilers internal expansion vessel?