genius me!
 

because i'm a ham-fisted twat i dropped a socket & extension into the bulkhead underneath the cabin air filters as i was doing up the cover after changing the filters. i removed the filter box again, took off the duct but still can't get at it. there's a wall separating the bulkhead area from the actual engine bay tho. will this thing drop out on its own? is it likely to cause damage? do i have to strip everything out of whole bulkhead area?

stand down red alert
a couple of coffees later i stuck my arm down there again and, sacrificing a fair bit of skin, i got it out.