I always believe that it is appropriate to dress for an afternoon / evening wedding in something smart, but not overly formal. To bridge the gap, why not invest in a spangly cardigan or wrap which you can use to cover your shoulders for the afternoon ceremony, and then you can leave it in the car / cloakroom, and look suitable for the evening. You may wonder how I know all this? I have two sisters, threee daughters, and i worked as the only man with twenty-two ladies for about six years - you learn a lot!