Its something to do with the fact I have the bolt on type
Quote from Mitz...
"Hi David, the bolt on type spacers were hitting the back of the wheel, as there wasn’t sufficient room on the wheel to allow the original wheel studs to fit. As a result of this, you would need the slip on spacers which come with longer wheel studs."
Not sure what is meant by "slip on spacers with longer studs" - I suspect that is meant to say you need to add slip spacers as well as the 20mm spacers so that the original studs do not hit the inside of the wheel as they are normally 25mm long. I had this issue on the 370 which with Andy's help (Kudos) we resolved - see this thread, with pics to help the explanation.
viewtopic.php?f=103&t=34297&hilit=spacers
If you use 25mm spacers there would not be that problem, but then you need to make sure you get at least full 9* turns of the nuts holding the spacers onto the original studs.
* I think it is 9 from memory.