I got married at The Roslin just 8 weeks ago, as I only lived round the corner (since moved abroad) so it will always have a special place in my heart. The food is lovely, as are the staff. I wouldn't worry about wearing something warm, they have heaters outside if its gets cold. In fact the smokers at the wedding were out there without any issue (and you will remember how cold it was back then). Hope you have a lovely time. Makes me feel a bit homesick actually!