Our resident ginsarian hasn't tried this one yet, but here's what Handmade Cocktail have to say:

Known for being a classy beverage, The Handmade Cocktail Company use the very best copper pot-still to bring The Gin Martini into the comfort of your own home. Using the technique of mixing the vermouth with the gin before bottling results makes it easier to serve and adds the benefit of maintaining freshness for longer. The concept of the vintage gin collection is you get all the ingredients you need to make superior cocktail, all you have to do is pour it over ice, stir, then strain off into the glass and then for the final touch include a garnish of your choice, with the addition of each bottle having a stamp showing the year it was manufactured.