What is interesting about novels like The Day of the Triffids and their ilk is that the focus is on the creation of a dystopian world which has replaced the order and normalcy of our world today with chaos and disorder. One of the problems with many dystopias is that this emphasis is only achieved at the expense of developing convincing characters. Often characters are left rather vague and undefined. You might like to consider The Handmaid's Tale by Margaret Atwood as an example of this.
However, having given this caveat, it is clear that the character of Bill Masen certainly develops throughout the course of this excellent novel, as he is involved in various groups of post-disaster humans, which allows him to comment critically on the various groups and responses of humans to the disaster that has befallen humanity. For example, the groups of Michael Beadley and Wilfred Coker have a very different response to their situation and their responsibility towards blind humans. However, gradually we see Bill Masen developing into a character who is not happy to be merely led but wants to lead, and this is seen in his desire to search for Playton and his decision to run the farm with her in Sussex. He has gradually transformed into a leader.