@fobose , not AC but a different kind of ventilation. It's the hole for the vent from an en-suite shower that's been out of commission ever since I discovered the previous owners of this house had the extraction fan just blowing in to the loft space. Then they covered it with insulation and then covered it with loft boards. Numpties! All that moist air just condensing but I reckon I got it in time. I'll post some more pictures tomorrow once I'm finished.
Rather than lifting the loft boards (nailed in you see ) I just routed out a hole in the boards and thankfully my Routabout saved me from a bit of a shock. Not really, I knew the cable was there and that the router's bit wouldn't hit it, but still, I hate it when electricians are lazy and notch the beams for the cables rather than make a hole in the middle of the beam.
nice clean cut