It would be a good idea to put the tree into a larger pot and then maybe put it outside in May 2007. The other option is to pot it on into another larger plastic pot say 30 litres and then in May plunge (leave the tree in the pot and bury it in the ground) the tree in the place that you wish to plant it. Remember though that you must water the tree if you leave it the pot and also make good provision for drainage from the bottom of the pot i.e. put a 3-6cm layer of gravel in the bottom of the plunge hole. Then in the autumn next year you could bring the tree into a sheltered position for the winter and plant it out proper in May 2008.