Hi Emma
http://www.australia.org.uk/lhlh/fil...idencing%2epdf
The link above is to the visa evidencing section of the Australian High Commission website in London. You could try e-mailing them with your query.
In the letter granting my mother's visa last year, the CO did not say anything about having to get the visa evidenced by any particular date. I think you can do this bit whenever you want.
Does this help?
I think your idea is absolutely brilliant. It will be the journey of a lifetime and it is a terrific way to head in the direction of Oz. How are you going to do the last bit, though? Are you going to get a tramp steamer from Singapore or somewhere? I think that would be a fantastic way to do the last leg to Australia, myself.
Best of luck and you should write a book about your journey so make sure to keep a diary as you go.
Gill