GB2.....I'd say the bike has had little use, if the stand stops are anything to go by. Most show severe wear indents, the result being the bike sits lower as the stand legs end up too far forwards when on the stand and a hernia inducing up and over lift to get it off the stand.
Unless the frame is damaged or you intend a hard tail conversion, there is no need to split front and rear frame sections, maybe why the steel joining sleeve does not appear in the parts list. I've made them, just a tube turned down to fit the lugs. If you are determined to continue, expect hassle, they are invariably well set in their ways. It is a thin walled tube, a draw bolt, spacers and heat rather than violence should do it, but in the grand scheme of things, a somewhat unnecessary exercise as the frame looks a good one.
Drilling out a sheared off saddle nose mounting also presented a challenge, no Chinesium here!
Swarft.