So update: Thankfully there's only one leak in the system, and that's a slight gap between the sink bottom and the waste that I didn't properly cover with silicon; everything else is jank but otherwise okay.
However I do need to go out AGAIN and buy another appliance inlet hose as the one for the washing machine is really too short for when it's out of the alcove and is going to put a lot of pressure on the pipework if I need to get it out to turn the water off.
When we eventually redo the kitchen I want the appliance water points to be easily accessible rather than behind the appliances themselves.