We also work in Teesside and can assure you guys that I will not let the tds go above 2.
I have always put it down to the salt sea winds off the sea in Saltburn and Skelton, but Redcar also experiences polution from industry that does mean that we have to spend more time scrubbing and rinsing to ensure a good clean. We only do a handful of houses in Redcar and they are all the same.
I'm sure some can clean to a high standard with a higher tds water, and have long held the belief that its what the dissolved solids are in the water that determines what you can and cannot get away with. Location also has a lot to do with it as well as the time of the year.