I have a reoccuring problem with my lemon tart. I pull it out of the oven and it looks perfect. When it cools I carefully take it out of the fluted flan tin (mostly I used a knife and make sure the edges have come away from the tin first) and inevitably a crack appears right around the edge of the tart seperating the filling from the pastry. It is driving me crazy as it doesn't always do it and I can't seem to work out what I am doing for this to happen. Any ideas???