I've no experience of slipper clutches but I doubt that that friction plate will be pressing directly against the basket. I've just watched a video of a slipper clutch being fitted to a Duc and the hub had an integral smooth plate at the bottom, which the first loose plate, which was a friction plate, would be up against.
Top man! U were right! So I have 2mm plate out and adjusted the lever. 25 miles later still good. Thank you all. I presume now it starts wearing I will be refitting the 2mm plate.