Modular Downlight Single, Dual or Triple - White or Aluminium

What is the recess depth required in the ceiling to fit these? Do you have stock if the triple bulb fitting?
most of the depth comes from the lamp and lampholder. We would suggest a depth of 100mm. As we write the white version is on back order - 2-3 weeks

What holds the light up, is the box screwed to the ceiling joists?
This fitting is like a downlight and has spring clips to hold it in place.

Are these light fittings suitable with dimmable GU10 LED's?
Yes the mains versions can use dimmable GU10 LED lamps.

Is there a 240V mains supplied version instead of the low voltage type?
This is a mains fitting. GU10 based lamps. Can use Halogen or LED lamps.

What is the IP rating on these lights please?
The fixtures are open so have a IP rating of IP20 or IP21. Fine for most applications but not suitable for bathrooms or exterior locations.

How do I change a lamp in this design? I cannot find a way to unscrew the front rim. Taking the whole unit out of the ceiling seems like a horrible option to me.
There is no need to remove the fixture from the ceiling - the bezel in front of the lamp pulls off (it is held in by a spring steel. Just hold the housing and pull the bezel infront of the lamp off.