Well Bill, I'm pretty traditional and my first thought is that since a Utilikilt is anything but that, whatever you do won't end up being traditional so it wouldn't much matter.
From my point of view, a scarf or sash is women's wear and you're better with neither than with the wrong thing, and since a fly plaid is pretty extreme in formality, it won't go with a Utilikilt anyway.
Now on this forum, no matter what is said, there will be someone who will disagree, but frankly my choice is generally tartan 8 yd with a tweed jacket (vest optional) proper hose coloured to tone in with the tartan but not to match it, no fly plaid, and a fairly plain sporran with plain Oxford or Brogue shoes, not Gillie brogues. For a shirt, either a white shirt and tie or if no jacket, then a tattersall shirt.
Have a look at this thread for some variations on a theme; you can't go wrong here: http://www.xmarksthescot.com/forum/f...-attire-46888/ These guys know what they're doing.
Here's one of my own casual photos:

and here are my son and I dressed for a formal event:

Our hose in this one are way over the top for the event. My son was the man of the evening, so he's dressed up more than I.
Cheers!
Rev'd Father Bill White: Mostly retired Parish Priest & former Elementary Headmaster. Lover of God, dogs, most people, joy, tradition, humour & clarity. Legion Padre, theologian, teacher, philosopher, linguist, encourager of hearts & souls & a firm believer in dignity, decency, & duty. A proud Canadian Sinclair with solid Welsh and other heritage.
Bookmarks