I trawlled the web a few weeks ago to get a recipe for mint sauce, tried one and made a complete mess of it.
I never actually got tround to trying it again, (I will have to soon, the mint's about to take over!) but my advice would be to add each ingredient a really small amount at a time, that way it won't turn out too sweet, too vinegary or too runny!
Good luck, hope it turns out better than mine!
This is the way my mum always made mint sauce. Finely chop enough mint to half fill a warm glass (1/2 pt tumbler) put in about two teaspoons of sugar pour over enough boiling water to disolve down the sugar and just cover the mint (which will shrink down a bit) and then top with a good light vinigar (wine, cider or light malt) to taste it is really nice with new potatoes and lamb but a different to what you buy in a jar!