Thanks for the reply jfobart.
What I ended up doing, and in the world of shade tree mechanics such a method would be termed a "bodge", was select and cut the bottom half of the door, move it to a new layer and align and skew it into place at the top of the door, then lower that layers opacity way down, to about 40% I believe.
All was needed then was to erase the door handle from the "mask" layer, and I ended up with a, well, less obvious camera flash, looks more like a reflection from a tamer light source, good enough for the web, I hope. I can see where it's been "bodged", but hopefully if your not looking for it you won't see it.
So there's no hidden feature in Photoshop for that type of situation then, oh well, will be some day I guess.