When inspecting my upstairs which I rent out I noticed that the bathroom basin has been replaced with a completely different basin which doesn't match the suite. I e mailed the tenants to complain about them doing this without telling me and they are saying that they thought they were doing me a favour because the original basin (which was only four years old) had 'developed a crack'. This seems strange to me as I have never known a basin to do this before without being knocked or sat on or something.

Unless there was some manufactors defect no they don't just crack. It would have had to be something done by someone

I've seen formed basins crack at the drain from being over torqued when installed - also seen them surface bubble in that spot - usually caused by hair dye products or spilling nail polish remover.