junction box size and location in ceiling void.

I am currently in the position of rewiring several sets of downlighters for a large kitchen / diner / living area.


There is currently a pretty awful setup of 'choc' connectors screwed to a joist and then a cats nest of cabling to this from/to the various zones of lights.


Clearly going forwards this is not acceptable and given that the whole ceiling is down at present I am keen to put this right.


I am planning on terminating the cabling in a junction box. All of the 1mm2 t&e is now installed and I would like to use the wagobox XL, however the joists are only 170mm in depth and pretty much any junction box which offers enough space to comfortably perform all the connections required would put the junction box within 50mm of the top or bottom of the joist. 


Could someone point me in the right direction to understanding the rules of junction box placement in this context? I cannot see anything in my on site guide.


Many thanks


James