The Halfords nearest to me (Broadbridge Heath near Horsham) had Black Sapphire in a rattle can on the shelf. I was happy with it. You need the lacquer as well, which Halfords also had.
Here's the BMW paints (with a Citroen intruder!) on their website:
http://www.halfords.com/webapp/wcs/stores/servlet/SearchCmd?srch=paint+spray&catalogId=10151&action=listrefine&tabNo=1&categoryId=-1&langId=-1&storeId=10001&qcon=fh_location%3d%252F%252Fcatalog_10151%252Fen_GB%252Fcategories%253C%257Bcatalog_10151_c165744%257D%252Fcategories%253C%257Bcatalog_10151_c165744_c318480%257D%252Fcategories%253C%257Bcatalog_10151_c165744_c318480_c165505%257D%252Fattr_f770e3d3%253E%257Bav_8a819209%257D%26fh_refview%3dlister%26fh_refpath%3dfacet_41%26fh_reffacet%3dattr_f770e3d3%26channel%3ddesktop