If anyone's interested, Motorsport-Tools are selling Ribe keys (you know, those funny socket screws on the cam cover) of various types on E-Bay http://stores.ebay.co.uk/Motorsport-Tools-UK?_nkw=ribe+&submit=.
I just ordered a 7 mm Ribe bit which is ideal for those difficult to reach screws on the timing belt cover! Cost £2.99