I have a white ceramic washbasin that's perfect except for a chip
through the glaze where some clown has dropped a heavy object on it.

The chip is about 2mm square and maybe 1.5mm deep. It's about 100mm
left of the plug hole, so would be below the water line.

AFAIK there are at least a couple of products that claim to be able to
repair this kind of damage.

e.g.

http://www.boundarybathrooms.co.uk/W...ctDetail=78376
http://www.boundarybathrooms.co.uk/W...ctDetail=78378

Does anyone have any experience of using these or similar?

Can they really achieve an "invisible" repair?

Does the repair last?

Is filling then painting rather than just filling more appropriate for a
chip this size?

Or would repairing it be a waste of time?