Most weddings aren't that formal any more and outfits are as much about wearing something that is suitable for the entire day, not just the service. A strapless silk dress sounds perfect but some churches frown upon bare shoulders (even for brides - my local Catholic church has banned them). Also, with it being March it's bound to be a bit chilly. A jacket with a glam dress might look too starchy and mother-of-the-bride. But pashminas are great for covering up and keeping warm in a v. glam way (and can be dead cheap to buy if you hunt around) and when you get to the reception you have your fab party dress on underneath for all that champagne drinking and dancing! There are also lots of cute cardigans in the shops right now or, if you don't mind getting a bit chilly, a shrug will cover up your shoulders while not hiding your dress (eBay has loads of them). Hope you have a great time!