Lactic acid well get yah well below 5.16ESB is mostly a London beer, rather than a Burton. So not a chance in hell.
There is lots said about Burton water, a fair fraction of it is BS or at a minimum not very historically accurate. Another point worth considering is that brewers have always modified their water, boiling, slaking or aerating all act to change the water that brewers get handed.
In Burton there were over a hundred years three separate water sources used, the river, shallow wells and deep wells (referred to as above and below the Marl - a clay layer that separates two water tables), all very different, which one made Burton famous?
Would need to see it to believe a reading of 5.16pH to believe it, there simply isn't enough phosphate in a grain bill to get you down that low. Mind you there is some work being done on mashing lower than that (<5.1pH), kills of Lipoxygenase one of the big beer staling processes. Probably of more interest to low body lightly hopped yellow pissy fizzy than bigger beers.
Mate do a bit more reading up on Water Chemistry, have a look at some of the better calculators and see what they recommend for an ESB, just don't go adding 700 ppm of Sulphate and expect anything other than a very minerally beer.
Avoid people that tell you to add carbonate to anything that isn't black as and if you really care get a pH meter.
I targeted Amber Dry on bru'n water, which is about 150 sulphate to 50 chloride. I liked it. Too much so4 can give a minerally bite which some people enjoy in beers, I don't.Not sure if it will actually get to 5.13 this was just going off the software, which is what I have let do all my water calculations based off my source water report.
I get the basics of water chemistry (Sulphate to chloride ratio etc) and I just let the software do the rest and add what it tells me to get to the target profile. Not at the stage of getting a PH meter yet. I figure Brewfather will get me close enough.
Reading this and other posts I think I will steer clear of the burton Profile, 700ppm seems ridiculous compared to any other profile I’ve used. What would you recommend for an ESB?