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
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