Downlights not sitting snugly

[Taylortwocities]

As above. Your basic problem is that you seem to have two lengths of (is it 1.5mm?) T&E on the terminal box.
The stiffness of the cable is pulling the light out of square.

Just try fitting one DL with no cables attached. If it fits properly then the cables are the problem.
you’ll need to terminate those chunky cables in another JB and just run a single bit of thinnish flex to the DLs.

 

[robinbanks]

And if you have e.g. 300mm of insulation/acoustic wool resting on the light and cables, install something to lift it up so it isn't weighing on it. That might be something as simple as a piece of batten an inch longer than the height of the light, stood up inside the ceiling so it lifts the wool next to the hole

 

[Jupiter01]

UPDATE - I think the rigidity of cables was a factor but I first tried @robinbanks suggestion of cutting a piece of wood, slightly longer than the height of the light and slipped it to the side of the hole to hold up the insulation. Once this was in place, the light snapped into place.

Thanks for your help with this.