£43 is outrangeous - it's just a cable. Try E-Bay. For example,
http://www.ebay.co.uk/itm/BMW-iPod-iPhone-Y-Cable-USB-to-AUX-Interface-Lead-fits-2007-to-Present-/190631518734?pt=UK_MP3_Player_Accessories_Cables_Adapters&hash=item2c6286220e
I have on in my 5 series (not this specific cable), works fine.