Help. How did it happen? We only tiled last year and the tile is on top of Marmox boards and we used flexible adhesive and grout. The marmox is on wood so it was stuck down as well as screwed down. There was a different bit of floor near to where the crack happened, that was chipboard but it was level with the floorboards. We haven't had the underfloor heating on for ages and are now worried incase it's damaged it. It's just a hairline crack so far - will it get worse?
It's very unlikely to get worse but neither can it be easily fixed. This is what happens when the sub-floor comprises more than one type of material - they expand and contract at different rates as the house gets hotter and colder and if whatever on top of it is firmly fixed, if there's no flexibility it will pull it apart.
The only way to fix this properly is to take have it all laid on one type of substrate or include an expansion joint between the two types of base. However, after a year, there's really not much chance of it worsening so you might as well just re-grout it. Can you remember whether the edge of a sheet of marmox lined up with the perimeter of this other type of wood? If you ever replace this, always try and overlap these areas.
Regarding the UFH; if it works then its un-damaged, if it doesn't then it is. Even if it’s damaged, the UFH will not be dangerous - it just won't work. However, there is such flexibility in these UFH cables that I think it would take an awful lot more than a hairline crack to break it.