Well, further update time: Having brewed at the test plant (10bbl system), they mash at 2.5:1 Water:Malt (pretty standard). Looks like Bi-carb is needed for darker beers otherwise the pH goes really low. Infact, with some basic CaCl2 and CaSO4 additions the pH goes pretty low on pale beers as well (a few times, down to 5.20 which has meant I've had to make a few changes; moving some salts to the boil etc). This can only lead me to believe an overly acidic mash is where the weird 'twang' was coming from.... or something else haha. I'm pretty sure my water is the same as at the brewery; or very close at least and the pH numbers we're getting there aren't anywhere near as high as the easy water calc is suggesting.
Suppose the only way to know for sure would be to test my homebrew mash... but I've not done it for a while... maybe if I get a sample of my water, take it to the brewery, measure a small sample and mash it and test it there would be a good option.