My further 2c (probably up to 14c by now?) - warning, anecdotal evidence following:
I brewed an ESB with my Brisbane water,
without adding gypsum and diluting to approx
9L/kg in the mash (my stuffup on brewday):
78% MO
8.5% demerara sugar
6.4% medium crystal (Thomas F)
1.6% roast barley
Hopped to 35IBU with Challenger at 60min, plus some Willamette at 15min
Wyeast 1275.
It was quite astringent, obscenely so in the pre-ferm sample. It improved somewhat mid fermentation, but was bag again towards the end (typical) at 1.013 or thereabouts. However, at 1.010 now it is quite smooth, but still has a definite hint of astringence, astringency, astringentness and maybe even astringentism. Probably astringency. Did pH play a part? I didn't measure so I can't totally say (hence the anecdotal).
A day later, brewing my extra-pale black IPA with 2g/kg gypsum, max dilution was 4.5L/kg in the mash:
88% Weyermann vienna
6.8% Weyermann CaraMunich I
4.5% wheat
1.057, 50IBU
9 IBU Simcoe at 60min
8.5IBU Simcoe at 50min
8IBU Simcoe at 40min
7IBU Amarillo at 30min
6.2IBU Amarillo at 25min
5.5IBU Amarillo at 20min
3IBU Cascade at 15min
1-2IBU Cascade at 5min
Smooth as the buttocks of a young child at a tasting at 1.023 - obviously will get drier but I can't see it being an issue at all in a week. Again, did not measure pH but even the fairly chunky volume of hops had the pre-fermentation sample tasting wonderful.
Obviously different hops and different profile, but the 50IBU came across significantly smoother than the 35IBU. I have used Challenger and Northdown in a previous ESB that was beautiful (I always do 30-35IBU for some reason for these beers), but had what I would describe as a 'bite' on the tongue rather than an astringent note, and was dosed at 1.5-2g/kg with gypsum. Same water, guarantee the water/grain wasn't as diluted as it was a fly sparge and a moderate boil only. The yeast in that case was Whitelabs Burton Ale, but it was a glowing bright beer.
But, I can probably throw all this out the door as I didn't measure pH in either case. There are a few definite pre-cursors of higher than ideal pH - poor mash conversion (some starch in the mash), astringency, dark cloudy wort.
Very anecdotal, but if somebody out there has kept better records and noticed astringent bitterness correlated with high pH, it might be something to steer away from in future batches. I will be brewing the ESB again soon - was happy with the recipe and love the yeast - so I will be curious how I go with gypsum dosing and a less dilute mash. I will just have to brew it within the next couple of months - and it will still be anecdotal until I measure two examples.
EDIT: corrected gypsum to 2g/kg from 2g/L