I recently bought a 2010 R56 Mini One, I'd like to fit a body kit including the rear valance but I didn't find any aftermarket exhaust systems which would give me twin tail pipes in the centre (rather than single pipe on one side) any suggestions on what exhaust system or estimates on cost? will original Cooper S system fit?

You could ring up Lohen and ask if the borla system for the Cooper S would be compatible with your Mini One?

lohen.co.uk - Track & Fast Road Performance for your New Mini - Online Shop (http://www.lohen.co.uk/shop/proddetail.php?prod=BorlaCatBackCooperS)

This may help with ideas (although just noticed you have R56.......not R50):
MINI COOPER :: North American Motoring - Your MC (non-S) exhaust reference thread (http://www.northamericanmotoring.com/forums/cooper-non-s/151767-your-mc-non-s-exhaust-reference-thread.html)

On a previous (2003) Cooper i had a Cooper S exhaust fitted so i'm hoping on the 2007my i can do the same?
On the 2003 the black plastic carrier thing underneath the boot had to come off and the heatshield needed trimming. The plastic bumper trim from a S model can then replace the Cooper trim so you have a cutout for the tailpipes

I've yet to see the exhaust differences on the 2007 but hope it would be as easy?