Wow, thanks for all the comments i'll try and explain (and remember) what i did with it.
I replaced the sky (yes i photograph skies now, sad) using the magic wand on the old grey,
bland sky. I copied across the moon and changed the blending mode to 'screen'. Flattened
all the layers, copied the green channel and pasted that over the colour background, which
gives you a new mono layer.
Change the blending mode of that to 'overlay' which will give you the gothic look. Flatten
the layers and duplicate the now background so you have two layers of the same pic. Then i
used the lens flare filter on the streetlamp and reduced the opacity of that layer until
it looked about right.
Flattened it and tweaked the colour balance green and red until i was happy with the colour
I hope that made some kind of sense, i'm afraid 'doing' it is easier than explaining it..