Houndog

Hi I am new to this forum, having just bought an 11 plate diesel automatic Nemo, it seems like a good forum to be a part of. I have only had the car for two weeks but have found sporadically the rear door refuses to open. All the other doors open and lock just fine. It doesnt happen all the time, in fact it did it twice in the first 3 days of owning it, went a full week when it behaved itself until today and it has remained firmly shut for the whole day. I have tried opening it manually with the ignition key which worked once and then failed to open it. There isnt a key lock in the rear door, only on the drivers door. Any one have any ideas?