Well, I can't say I perfected the recipe, although I've come close to the flavor of the original stuff, before Heineken closed the brewery and moved production.
The 'original' recipe was brewed at high gravity and diluted at packing. It used pale, stout malt, roasted barley, wheat, and wheat/barley syrup. Stout malt is a relatively low color base malt with more enzymatic potential. The wheat is used as cheap extract and the barley/wheat syrup were added in the kettle to increase yield. It is worth noting that these syrups have a similar fermentability to malt but provide no color and flavor; sugar is not a good substitute, although it is not really needed anyways. Hops were a mix of German and UK varieties, but all that really matters is getting a clean bitterness. Yeast is a rather neutral strain that has high attenuation and flocculation.
My current recipe and closest is 72.5% 2-Row, 15% Wheat Malt, 10% Roasted Barley, and 2.5% Black Patent. The roast character was something I always had a hard time matching and eventually found that Breiss RB (300L) with a small amount of BP malt provides a similar flavor. A 60:40 blend of UK roast and chocolate malt also works, but isn't quite the same. O.G: is 1.039 and F.G: is 1.007. I use moderately hard water profile, diluting with RO to around 120 ppm ALK and target 100 ppm Ca for final wort. Mash pH around 5.2. Calcium chloride is used, no sulfate, and chlorides are around 75 ppm mash. For hops, I prefer Northdown and Goldings, but any clean bittering hop to 30-35 IBU works. Not looking for much hop character. As for yeast, I now use the actual Beamish yeast with pure O2, but you can easily substitute it with a clean and attenuative strain. Pacman has worked very well for me, although WY1084, WY1335, WY1056, or even dry Nottingham is acceptable. Just avoid any fruity strains and keep temperatures relatively low. IMO, the key to this beer is achieving a dry, smooth roast character with a very clean fermentation profile. Good luck.