It is a simplified Singapore Sling as Dick made at the Zanzabar club, built to be a truly "British" cocktail, but you should really hear it from the man himself. You can do just that in this excellent video interview from DRÉ Masso's website: www.ginandtales.com
Since its creation, it has seemed to work its way onto nearly every cocktail menu in London at some point or another, and it remains on of the city's favorite warm weather coolers. A modern classic from London's "godfather" of cocktails.
2 ounces (60 ml) Plymouth gin
1 ounce (30 ml) lemon juice
2 teaspoons (10 ml) simple syrup
1/2 ounce (15 ml) (nice quality) cremé de mûre
Combine the gin, lemon juice and simple syrup and shake well with ice, then strain into an old fashioned glass filled with crushed ice. Add the cremé de mûre and stir. Garnish with a blackberry and a slice of lemon.
For the crushed ice in your serving glass, break out the swing-away ice crusher (worth picking up on ebay if you do not have one). You can also crush your ice in a lewis bag with great results.
What we have here is a gin sour, sans the optional egg, wonderfully laced with the cremé de mûre.
The drink is very gin forward, which is why I think that Dick recommends the smooth and subtle Plymouth gin. For you gin lovers out there though, don't be afraid to use a nice bold gin as they work very well here too. (Not that I need to tell you).
It's also worth noting that for non-gin drinkers this is a very approachable drink and a great way to introduce yourself to the spirit, especially if you go with a mild and smooth gin like Plymouth.
The Massenez cremé de mûre is a very nice liqueur, and one of the better brands available here in the states. The blackberry flavor is somehow bold and subtle at the same time.
This drink really develops well as it sits, becoming more and more enjoyable as you get on with it. That is, if you can get it to last that long.