Martin
Thanks for your response. You are correct when you say that the hexagonal section has split. The cap that goes into it has a very tight fit and simply blows that thin section apart when the plastic ages. A glue would only work up until the time that the cap need to be forced back in and then the tower splits wide open again.
When the hexagonal section has split and expanded, the sliding part is no longer able to slide smoothly. If the tape is put in, in a way that forces the sliding section down, it is unable to spring back and lock in with the cassette. Thereafter, rewind will simply not work. My solution at the moment is to line up the cassette correctly before I put it in - then everything works OK. As you say, you could glue it in place and that might just force the spool to twist when it loads.
In the meantime, I will bide my time and see if a spare one turns up.
Regards
Geoff