Not as such! James and Sarah will have already been married for two years at the time of the event, so it's going to be more like a wedding reception. There'll be food, some drink and chance to mingle and see folk you may not have seen for some time.
Either Arlesey, Letchworth, Hitchin, or Baldock. Unfortunately, Stotfold isn't served directly by a train line, so you'll need to sort a lift for the final leg.
Smart casual, think of what you'd like to be wearing in a photograph. James and Sarah show an excellent example on the home page of this website.