Special Bitter is a sub-section of English Pale Ale and I would treat my water as if I was brewing a pale ale.
I use my tap water and reduce the alkalinity to around 30ppm. At around 11 SRM, there will only be a small amount of crystal / speciality malts. Many English pales/bitters use an all pale malt grist and the colour is derived from kettle darkening, the use of brewers caramel or a tiny amount of black malt.
Mineral content I think is very subjective, I usually look for 200-250ppm sulphate and 60-70ppm chloride; the general consensus being the sulphate will enhance the hoppiness.
That said, there was a big discussion recently on a UK based forum that I frequent regarding Timothy Taylor 'Landlord' ......a classic English pale ale / special bitter that is known for a hoppy character.......and whilst the brewery are very secretive about giving any information on recipes, enough was pieced together to come to the conclusion that their water is probably biased towards chloride.
I hope this is of some help to you.
BTW, what is your intended grist and hopping for the beer?? I would be interested to see your take.
Have you chosen a yeast? The WYeast 1469 is fabulous for this style.