Because of the large amount of calcium (and the magnesium) added to the water, its common to need to add alkalinity to the mashing water to avoid pushing the pH down too far with the Pale Ale profile. This is the result of the Residual Alkalinity effect. An important fact to remember, is that the native water of Burton also has a huge amount of alkalinity, but the huge calcium and magnesium contents result in that water having its RA near zero.
Don't rely on chalk to provide your alkalinity. Use the baking soda exclusively for that.