Some thoughts from the peanut gallery.
The objective is:i want a smooth, semi-sweet but balanced, milk to baking chocolate flavoured beer of about 5%
AFAIK, Young's Double Chocolate is a Milk (or cream) Stout. If that's your intent, 1 lb (for 5 Gal) of Lactose will be needed. Young's is not too sweet in my experience.
The invert will dry the beer somewhat. If I understand the objective correctly, I wonder how this fits with "semi-sweet." Of course when adding Lactose and using that yeast, it may end up being too sweet. Its all about the balance you want.
The color seems too light for Young's, so increasing the amount of chocolate malts (450L) will help. But when increasing chocolate malts, my beers tend to get a coffee taste, which may or may not be desirable.
Alternatively, consider using up to about 5% Simpsons Black Malt(600L) along with some Chocolate malt. From Simpson's website, "Our Black Malt is excellent for darkening beer colour without imparting too much astringency or roast characteristics. The flavour is surprisingly neutral, with a clean dryness that makes it an incredibly versatile product."
This sounds like the flavor matches your objective while adding some color to meet the style (if that's important). I have found it tends to darken the foam, which you may or may not enjoy. Carafa malt might work as well, if that's what you have on hand.
Dave Taylor's comments on the chocolate match my experiences. Even when roasting the cacao nibs, they don't add much chocolate flavor. Adding a vanilla bean with the cocoa powder sounds strange, but I find it tends to accentuate the chocolate flavor.
A similar beer is on my list for December. We really enjoy it.