They have them here but not for 50p+ p&p
http://www.ebay.co.uk/itm/Genuine-Ford-Puma-Front-Wiper-Arm-Bolt-Cover-X1-/220748691284?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3365a5b754" onclick="window.open(this.href);return false;
Theres also a cover off another car that fits a vauxhall something or other I think it was, someone who remembers will tell you in a bit no doubt.They were more like a quid.
You might be able to get the nut from Fords for 50p thats another way of skining the cat.
Dont try any old nut though I tried that but the proper one has a lip that stops you overtightening it.