Would you be able to use the existing focus locking wheel nuts on some aftermarket 18" alloys.

Or would it be worth just buying a whole new set such as :

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_195025_langId_-1_categoryId_262855#tab1

Assuming the correct size for the focus is

M12 x 1.5

i got a whole set of 20 nuts/locking nuts from taperpro

http://www.taperpro.co.uk/2012/

email them for a best price!

Pretty sure the Ford nuts won't fit aftermarket wheels, think the Ford ones may be wider at the base and so won't seat properly in the wheels. I only know this from past experience of taking aftermarket wheels off both my old Mondeo and my current Focus and swapping them for genuine Ford wheels, and the narrower nuts fitted to the aftermarket wheels didn't fit the base of the Ford wheels properly...hairy enough journey before I first noticed!

The opposite of what you're planning, but I'd assume the same principle applies in that they won't fit properly. I think I have two sets of nuts for aftermarket wheels at home as a result. Will check the condition of them if interested (think one set was grubby, the other were new but don't think any were lockers) but postage might be dear given the weight.

Have you got wheels in mind then or are you keeping schtum for now?!

I actually had a thought and the ones i was interested in actually come supplied with the relevant locking wheel nuts and mounting accessories. Helps to read the small print lol