Gale force draught

[cjard]

Well, you're gonna be spending a fortune on heating with a draughty immediately in the other side of the plasterboard, so you might as well get a few boards round the edge ripped out so you can see what is going on..

At the very least cut a few 75mm holes with a hole cutter and poke an iPhone up there in video mode with the light on (or off if there's a good chance of seeing daylight from some gaping hole) and take a look around.. you can also get drain inspection cameras on eBay for a few quid, plug n play USB thing with an inbuilt light

Keep the 75mm biscuits you cut out, remounting them is a simple matter of putting a stick of wood up there, screwing through the ceiling into either end, then screwing the biscuit up and whacking some filler/paint over it

 

[Doggit]

The builder insists it is correctly done, and doesn't want to know

If you have draughts coming in through the light fitting, then it can't be classified as correct. Was it done under a building control notice; in which case you could ask them for some advice. You may need to find someone that can prove the builder was wrong, and then send them a letter stating your evidence, and you might need to seek solicitors advice over how to proceed.

Once you've ascertained how the space above the ceiling is constructed, it may well be a case taking down the whole ceiling, and then determining how best to reconstruct it to the correct specification. You may need to batten out the rafters to lower the ceiling, and then put some more insulation in, and then use led downlighters, possibly with a fireproof cap to seal the holes in the insulation.