Wasted money. Better just give me 0.1 btc and i will save ur £300
Usb cables suks. You  must use a shielded lan cable.
      
With these things you will probably be able to use even a 15 meter cable.
http://www.ebay.co.uk/itm/USB-2-0-To-LAN-RJ45-Ethernet-10-100Mbps-Network-Card-Adapter-blue-for-PC-T5-/330988621895?pt=UK_Computing_Other_Computing_Networking&hash=item4d10759047
http://www.ebay.co.uk/itm/10M-Cat6e-Gigabit-RJ45-Internet-Ethernet-LAN-Network-Sky-Dream-Box-1000Mbps-Cabl-/271304441812?pt=UK_Computing_NetworkingCables_RL&hash=item3f2b013fd4
 £1.58 x 2 +  £3.09 = £6.25  
EDITED: wrong, its lan adapter u shold buy 2 male connectors (£2.25 + 0.49)
http://www.ebay.co.uk/itm/USB-2-0-Male-to-Female-CAT5-CAT5E-CAT6-RJ45-LAN-Extender-Extension-Adapter-Cable-/251478836310?pt=UK_Computing_USB_Cables&hash=item3a8d4e8456