Hi David..
Seems that I have the same "Seat Roller" problem as
you have.......The outer "RUBBER/SOFT PLASTIC"
 tyre type material has disintergratde, which allows the
seat to foul the main beam..... Have you been able to 
come up with a solution to the prob' yet?..perhaps we could confer!!........Regards Peter
                
 Peter..... "The Oldest XR3i Cabbier"
                
 April 2012