Hi fraser, yeah a non-watertight sink is not good! :)

It looks like you may have a few duplicated surfaces stack on top of each other in this model currently, like if I select a surface here you can see how it only partially displays:




That can make things more difficult for the booleans, you'll want to just delete some things like that.

Also it looks like the pieces do not quite touch in a simple manner, there is a slight separation and little skinny surface here:






Anyway, those are some of the problem areas - I'll take a look and see if I can help to clean this up.

- Michael