Thanks for the confirmation and drawing Graeme! (and great shots again Heinz!)
The remaining Hudsons in NZ also have the ventral gunner's position, (with the exception of one incomplete airframe at RNZAF Wigram). Judging by photo's, it looks pretty cramped, and being built as it is as a retractable one-piece framework, I don't think you would be able to escape from it (it's right next to the door in any case so you probably wouldn't need to). Maybe it was jettisonable though just in case, but I personally doubt it.
Does anyone know if these would be lowered before takeoff? I've only seen it like that in your airshow pics, and the restored RNZAF museum's NZ2013.
My guess is it would only be lowered when nearing enemy airspace, as it would probably knock a few good mph off the airspeed.