Last night while working out there, I heard quite a bit of banging and rustling, and I was pretty convinced it was outside the shed, so I blocked the hole (and did a pretty good job of it).
Turns out I was wrong.
The possum was inside after all.
And he certainly let me know it! When I went out tonight, there was stuff EVERYWHERE! The possum has obviously run along every flat surface in the workshop, knocking stuff all over the place (and leaving plenty of possum presents at the same time).
So I spent a good 30 minutes searching, fruitlessly.
Finally I gave up, and got the job done that I had gone out to do. Just before locking up, I checked on the blocked hole. Turns out I was wrong about that too. Despite really jamming a board in there, the possum has bent the wall of the shed itself, and has forced its way out through the hole it knew was there.
I reblocked the hole, and hopefully this will be the end of it. One possum = one big mess!
Stu's Shed 
They are messy little buggers i had one trapped in a room of my hoiday house for a month. It was dead by that tome of course but the mess was unbelivable. It looked as though there had been a barroom brawl in the room