Owen's answer is great and was invaluable.
This is a truly ghastly and testing job!
Two other points:
1. make sure you locate the seal on the outside ring around the drum (not the drum itself).
2. Inserting the spring to lock the seal onto the outside ring is tricky. Help is invaluable to hold the spring in position as you progress down the drum, pushing it into position (makes your fingers sore!0. You may need to use something like a bicycle tyre lever (or handle of a spoon) to slide the final section of spring into the slot. It should snap into place at the end.
I tilted the washing machine backwards to get better access, but it is very restricted working space.
Good luck...and be prepared to take your time. Took me a couple of hours.
Peter
January 2014