How long is building work usually guaranteed for?

Koicarp

You need to be certain what the cause of the problem was before spending anything re-tiling because if the ground is moving it'll go again unless you sort it.

Dan_Iggulden

And how would I diagnose the problem?

ormus

it maybe that the two different floors are moving at different rates.
hence the crack. as been said, an expansion joint should stop that event.

ie
on older houses with extensions, its common to have a wooden and a concrete floor joined together.