Two more comments:
a) The grey plastic bits at the end of the pull down bar. I found these damaged - meaning that cord had sawed its way thru the plastic. Best to complete that work, and bust the part in half. Then with a file smooth of both the bottom still attached bit, and then the bit which broke off. Glue these back together with Super Glue.
b) When replacing the strings, make suren that you use string of about the same diameter.
start with the black tensioner end. Knot the string, and apply a lighter to make the knot smaller and permanent.
having done ONE and removed the bootom end fixing, MEASURE the strings ... and mark where the small bits are on the end.
NOW you can cut the string and complete removal.
Carry on and finish as above.