There is now a small damp patch on the floor.
It's not so important that it needs to be dealt with now as they still have to move the washing machine to do the floor tiling next week. This could be because (a) one or more of the pipes isn't properly connected, (b) the new waste pipe is too narrow for the water pressure, or (c) the machine has developed a leak. It's possible that the inflow pipes aren't quite tight enough (though there's no leak where they connect to the house pipes, it would have to be where they connect to the machine - which seems less likely, I wouldn't have expected them to touch that end of things). The waste pipe hypothesis seems rather unlikely (but it's a worst-case scenario). The machine is 10 years old, already has a number of less-than-desirable features, and I would say there's at least a 50% chance it's the culprit. It could well have been leaking before without me noticing - it's only because the floor is completely bare that the damp patch is visible.
If replacing one cupboard door (which the installer has ordered) and discovering that I really do require a new washing machine are the worst things that have happened during the kitchen installation, I still think I'm not doing that bad.