You will be surprised how much chocolate flavor you can get with Thomas Fawcett chocolate malt. It's a United Kingdom malt and it is available from a lot of suppliers. It is also a 325 to 400L malt. I use it in my English Mild around 5.9% and it adds a great chocolate flavor. The key is the Thomas Fawcett malt. Other chocolate malts may not have the same results. You should give it a try and see for yourself. It's a great malt.
In my experience the key things are a low kilned chocolate malt, a dark kilned crystal malt such as special B, a high water (fruity) producing yeast and a sufficiently high FG to give a sweet finish.