Big thanks to everyone who posted about which ones fitted for them, it helped me make a decision.

I went for this one:
http://www.amazon.co.uk/gp/aw/d/B00H...ink+htc+one+m7

Fits perfectly in terms of width, and when it's flush at the bottom it covers about 1/3 of the bezel above. Nice and smooth on the edges too (rounded corners) which is obviously important for edge swiping in Sailfish!