Have you measured your mash pH with a similar grain bill? What pH are you aiming for with this and how far off would you be if you added nothing?
The other minerals in the bottled water are also important as they can affect pH and flavour so a mineral profile for the water would be good.
Step 1: Find out the sulphate, calcium, chloride, and zinc concentrations of your water. Sodium and magnesium can be ignored as long as they are not too high. Also find the pH (you have already) and the carbonate content/alkalinity (often expressed as CaCO3).
Step 2: Work out your mash pH, either by measuring or by plugging into a calculator such as EZ water calculator. Measure the pH of your actual mash to see how close the calculator comes..
Step 3: Drop out any unwanted products from your water such as excess chlorine, chloramine or carbonate. Depending on what the excess product is, you may be able to do this simply by preboiling or by using an activated carbon filter. Otherwise you may need Reverse Osmosis filter or to cut town water with bottled/spring water (or get a water tank and get the profile analysed from time to time).
Step 4: Get your calcium levels to at least 50 ppm (or mg per Litre). Get your sulphate and choride where you want it for the beer you are making (sulphate pushes hop profile, chloride pushes malt profile). Too much can be worse than none, avoid sulphate in dark beers.
Step 5: Taking into account your grain bill, water mineral content, volume of mash water and any calcium salt additions and calculate mash pH at room temperature. Aim for pH 5.2-5.8 at 20 degrees. Hotter will read lower.
Step 6: If your pH is too high with all the above accounted for, add some food grade acid - phosphoric or lactic are available from most good AG HB outlets. Calculate how much with the water calc and again measure the actual mash to see how close it is.
Step 7: Add some extra salts or acid to your sparge water if the pH is high. Otherwise add some extra salts to the kettle for flavour.
Step 8: Add some yeast nutrient (proper stuff) to the boil or zinc chloride to get your zinc levels up.
That's as basic as I can make it and doesn't include the reasoning behind anything. Haven't touched on raising pH with dark beers but you're looking at pale so that hopefully is a start.