Thank you all.
I tried PB's suggestion first, for a number of hours off and on. I even offered to make it a nice blanky bed from collapsed Commodore roof linings. No dice, I reckon it had smoked a point of meth and all it wanted to do was have a go.
Eventually - after basically giving up and asking for suggestions - Meth Possum crawled out and skanked off into the night.
He will get a treat for being a good chap in the end.
I was nearly at the stage of asking the parish priest to come and baptise it, because then it would only return for Easter and Christmas...