I just did a little research. There is not much info out there. I found an old add that said they use Norfolk barley and Beltring hops. I looked up Beltring hops, and Beltring is a village in Kent according to Wikipedia. It also says Whitbread used to own much of the area. I also quickly browsed my Michael Jackson books, and I did not find anything on pale ale, but it did say that they used Fuggles and East Kent Goldings when they tried to recreate some classic porter recipies. My guess would be that they used Fuggles for bittering and EK Goldings for flavor and aroma. This is what I use in most of my English ales. Sometimes I do use Styrian Goldings though. I think Wyeast 1098 is the Whitbread strain, and I have had some similar brews brewed with it. I don’t use it though because it loses its effectiveness to ferment maltriose (according to the late, great George Fix) and I have experienced that the couple of times I tried to repitch the yeast. If you are looking for the authentic flavor of the original, you should probably buy some English pale malt/Marris Otter, 10% English Crystal malt, a couple of ounces of black malt if you have it for some color, 30 IBU Fuggles, .5 Oz. EK Goldings for flavor and .5 Oz. EKG for aroma, Wyeast 1098. Ferment for 10 days at 65-68 degrees. Bottle or keg. I don’t remember this being a very minerally beer so don’t worry about trying to Burtonize your water. This one is brewed in London or somewhere in the U.S. I just remember it being dry, crisp, and maybe a bit woody. Just make sure your mash is 5.2-5.4 and you should be fine.
Don’t be afraid of the sulfate in a Burton Pale Ale. I heard a podcast with the late, great Greg Noonan that said he used up to 900 ppm of Sulfate when he brewed a pale ale. I really like that minerally profile though so you may want to back off if you don’t want some burning match character in your pale ale. If you go too low on your sulfate though, you can get some dish soap fruitiness/character from your hops. I have experience that too.
I have read that Double Diamond uses Styrian Goldings. I think I read that in the book about Burton Ales by Roger Protz.