no insulation in the void of my dormer

[samphiregrower]

I just bought my first flat, the building was originally a single storey with ridged dormer windows to the front. Not sure when it was split into 2 properties or when the flat roofed dormer went in at the back. I was removing plasterboard to put in a shower and found a low stone wall, covered in builder's rubbish, with a timber frame on top. It goes all the way from the airing cupboard, through the bathroom - behind the loo and bath, and into the kitchen- behind the sink and washing machine. The toilet waste pipe , and hot and cold pipes to the bath, basin and kitchen all run through it , below the level of the wall. There is a vent out below the kitchen window and the whole thing is enclosed at the top by a wide windowsill. There is no insulation in it, and as I'll be closing it off with multiboard and tile I want to get the insulation right. Access at the kitchen and cupboard end is problematic due to the hot water tank and built-in kitchen units.

Currently there is no central heating. I might consider gas as it's in nearby houses, or possibly infrared panels. Not sure if that will affect ventilation. Any advice for a novice?