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Brilliant! Thanks dude, this is making everything fall into place. I think the phrase I was missing is 'bottom entry', which is unusual for me. :D
So if I've not gone totally mad it seems something like THIS would actually do the whole job all in one, and I'd not need to worry whether or not the fitting on the arm is for a hose because its clearly designed for it.
Great shout on the diverter valve too. I had no idea they even existed.
ReekBlefs-
Yup, that looks like it would do the trick. Obviously check the dimensions to establish that the shower head will be high enough and far enough from the wall and that the hose will reach. Then just find some fittings that will secure it to whatever the wall is above your window, it's a long arm on a small plate so the leverage could be considerable if you accidentally pull on it. You may also want to check what you're drilling into above the window, because there may well be a metal lintel.
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bottom entry
ffm
Fox
Ah! I stand corrected, I hadn't spotted that rail. I assumed that you would put the showerhead over the long side of the bath, but having it over the tap-end makes more sense.
I don't know what you had in mind for "clipping" the shower head to the curtain rail. I can't think of a specific bit of hardware that would do that unless you wanted to cludge it together with a universal bike light mount, since the diameter of the shower rail will be much bigger than a shower riser.
One option would be to mount a bottom-entry (ooh-er!) shower arm above the window so it goes over the curtain rail. That would allow you to attach a flexible hose from the tap, although you'll need to check that the fitting on the arm is for a hose, rather than a compression fitting for a rigid pipe. You could even fit another diverter valve to the tap, so that you could have one hose to the overhead fitting and one hose to the handheld. I have this one and it works well.