Near as I can tell, modern Bass is brewed to different strengths depending on what the final package will be. Draught (and widget-can) Bass is brewed to an OG of 1.043, where bottled Bass is brewed to 5.1 ABV%.
At any rate, the grist is pale and caramel malts, probably with sugar to round out the fermentables. Hops are Challenger and Northdown. Me, I'd use a blend of both at all stages to round out the flavor. I can't definitively prove what hops are used, because bass.com is now devoted to some black & tan producing device instead of the quality of the ale. Idiots.
At the risk of gainsaying King Brian, while EKG are a fine choice as finishing hops for English ales, Bass is certainly not finished with Goldings. I can tell Goldings straight away, and there are none in Bass (at least the Bass available here in USA).
I think your grist is pretty well-designed. I'd go for 10% Crystal myself.