Thanks for that info gggGary. Much appreciated. My bike is apparently a June 1980. Presumably it will be fine for the 7 plate conversion.Plenty of info including pricing here. shipping to United Kingdom is quoted as $45 USD
https://www.xs650.com/threads/interest-in-8-plate-clutch-conversions.56486/
a late 1980 and up 6 plate will accept 7 alto plus one steel with no other changes needed.