I must ask... What size bolts/nuts/studs are being used here

Most Chevy wheels fit to 7/16"unf studs, most Ford wheels use 1/2"unf. Some later wide 5 VW to Chevy adapters use 1/2"unf though.

Have measured again and it's 17.1mm 

The nut size it 3/4af which doesn't give much of a shoulder for the washers to sit against...

Ok Hazel

Thats fine, 17.1 makes sense, its quite normal for a stud to be down on size, shall I ask
if its ok to get something done...

I think though, the washers need to be 17.20 + I/D though, I know this sounds silly, coz I have never actually
seen or fitted stuff like this.... 

But these washers are to be used 'under' the nut's.... ?

Yes, you are right, there isn't much of a contact face if the nuts are 19 A/F... ?

I don't know how 'normal' this is though...

Most US style sleeve nuts have 11/16" od and a 13/16" head. Maybe it's be better to get new nuts as well.

http://www.ebay.com/itm/New-single-7-16-RH-thread-Chrome-Mag-lug-nut-with-washer-Standard-mag-wheels-/230967105036?pt=Motors_Car_Truck_Parts_Accessories&hash=item35c6b6560c&vxp=mtr

To add to confusion, I inherited a set of cheesy crapanese locking wheel nuts from Chinebay,

they were 7/16" and stamped on the nut as 17mm as that was the spanner size needed